Add tests for uncovered code paths (#5581)

* Add tests for uncovered code paths

Cover code the test suite did not reach, found from the Coveralls report
of develop and a local coverage run of HEAD:

- dump() of every kind of value below the bound of the recursive descent
  (pretty-printed objects, binary values, discarded values, scalars), and
  flushes of the escape and write buffers mid-string and mid-binary
- the iterative comparison: objects with different keys, containers that
  are a prefix of each other, and elements that cannot be ordered, each
  both at the top level and below the nesting bound
- SAX handlers that stop at any event, including the end of a nested
  container, in the BSON, CBOR, MessagePack, UBJSON and BJData readers
- from_bson/cbor/msgpack/ubjson/bjdata returning a discarded value
  through the iterator and pointer overloads
- JSON Patch, diff, merge_patch and update(..., true) on ordered_json
- smaller gaps: get_allocator(), to_ubjson/to_bjdata into a string,
  value() with an unresolvable JSON pointer, integer/float comparison
  below the integer range and with negative fractions, conversion to a
  custom binary type, std::formatter::parse on a spec without '}',
  unescape() of a lone '~', and the callback parser's start_array()

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Cover more paths that were thought unreachable

- parse_float_fast() declining malformed or inexact input, called
  directly since the lexer only passes well-formed numbers to it
- a UTF-16 high surrogate followed by a unit above the low surrogates
- self-assignment of a const_iterator
- a truncated CBOR string read through non-contiguous iterators
- serializing a long double under the de_DE locale, which undoes the
  locale's decimal point and thousands separator
- values read from a binary format carrying no diagnostic positions,
  with and without a parser callback

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix the CI failures of the new coverage tests

- declare the self-assignment reference const (misc-const-correctness)
- expect the (/path) prefix that JSON_DIAGNOSTICS adds to the messages
  of the failing ordered_json patch operations

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Expect the byte range JSON_DIAGNOSTIC_POSITIONS adds to the patch errors

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Build the expected dump of the nested-object test with +=

clang-tidy (performance-inefficient-string-concatenation) reported the
chain of operator+ calls that assembled the expected indented output.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Compare the BJData and UBJSON test outputs byte by byte

Building a std::string from the byte vector converts each byte
implicitly, which -fsanitize=integer reports for bytes of 0x80 and
above (ci_test_clang_sanitizer).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-wstring.cpp

@@ -70,6 +70,8 @@ TEST_CASE("wide strings")
             CHECK_THROWS_WITH_AS(_ = json::parse(std::wstring{L'"', static_cast<wchar_t>(0xDC00), L'"'}), error_low_surrogate, json::parse_error&);
             // a high surrogate followed by a non-low-surrogate unit is invalid
             CHECK_THROWS_WITH_AS(_ = json::parse(std::wstring{L'"', static_cast<wchar_t>(0xD800), L'a', L'"'}), error_high_surrogate, json::parse_error&);
+            // ... also when the unit is above the low surrogates
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::wstring{L'"', static_cast<wchar_t>(0xD800), static_cast<wchar_t>(0xE000), L'"'}), error_high_surrogate, json::parse_error&);
             // a lone low surrogate must not swallow the following unit: pairing
             // it with any second unit would produce valid UTF-8, so the error
             // has to report an ill-formed byte at the surrogate's own position
@@ -99,6 +101,8 @@ TEST_CASE("wide strings")
             CHECK_THROWS_WITH_AS(_ = json::parse(std::u16string{u'"', 0xDC00, u'"'}), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"<U+0000>'", json::parse_error&);
             // a high surrogate followed by a non-low-surrogate unit is invalid
             CHECK_THROWS_WITH_AS(_ = json::parse(std::u16string{u'"', 0xD800, u'a', u'"'}), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"<U+0000>'", json::parse_error&);
+            // ... also when the unit is above the low surrogates
+            CHECK_THROWS_WITH_AS(_ = json::parse(std::u16string{u'"', 0xD800, 0xE000, u'"'}), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"<U+0000>'", json::parse_error&);
             // a lone low surrogate must not swallow the following unit: pairing
             // it with any second unit would produce valid UTF-8, so the error
             // has to report an ill-formed byte at the surrogate's own position