Improve unit testing (Part 1) (#3380)

* Refactor unit test creation

Add functions for creating tests and to supply test- and
standard-specific build settings.

Raises minimum CMake version to 3.13 in test directory.

json_test_add_test_for(
    <file>
    MAIN <main>
    [CXX_STANDARDS <version_number>...] [FORCE])

Given a <file> unit-foo.cpp, produces

    test-foo_cpp<version_number>

if C++ standard <version_number> is supported by the compiler and
thesource file contains JSON_HAS_CPP_<version_number>.  Use FORCE to
create the test regardless of the file containing
JSON_HAS_CPP_<version_number>.  Test targets are linked against <main>.
CXX_STANDARDS defaults to "11".

json_test_set_test_options(
    all|<tests>
    [CXX_STANDARDS all|<args>...]
    [COMPILE_DEFINITIONS <args>...]
    [COMPILE_FEATURES <args>...]
    [COMPILE_OPTIONS <args>...]
    [LINK_LIBRARIES <args>...]
    [LINK_OPTIONS <args>...])

Supply test- and standard-specific build settings.
Specify multiple tests using a list e.g., "test-foo;test-bar".

Must be called BEFORE the test is created.

* Use CMAKE_MODULE_PATH

* Don't undef some macros if JSON_TEST_KEEP_MACROS is defined

* Use JSON_TEST_KEEP_MACROS

Incidentally enables the regression tests for #2546 and #3070.

A CHECK_THROWS_WITH_AS in #3070 was disabled which is tracked in #3377
and a line in from_json(..., std_fs::path&) was marked with LCOV_EXCL_LINE.

* Add three-way comparison feature test macro

* Disable broken comparison if JSON_HAS_THREE_WAY_COMPARISON

* Fix redefinition of inline constexpr statics

Redelcaration of inline constexpr static data members in namespace scope
was deprecated in C++17. Fixes -Werror=deprecated compilation failures.

* Fix more test build failures due to missing noexcept

* CI: update cmake_flags test to use CMake 3.13 in test directory

Also change default for JSON_BuildTests option to depend on CMake
version.

* CI: turn *_CXXFLAGS into CMake lists

* CI: use JSON_TestStandards to set CXX_STANDARD

* CI: pass extra CXXFLAGS to standards tests

test/src/unit-comparison.cpp

@@ -32,10 +32,6 @@ SOFTWARE.
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
-#if (defined(__cplusplus) && __cplusplus >= 202002L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 202002L)
-    #define JSON_HAS_CPP_20
-#endif
-
 namespace
 {
 // helper function to check std::less<json::value_t>
@@ -211,7 +207,7 @@ TEST_CASE("lexicographical comparison operators")
                 {
                     // Skip comparing indicies 12 and 13, and 13 and 12 in C++20 pending fix
                     // See issue #3207
-#ifdef JSON_HAS_CPP_20
+#if defined(JSON_HAS_CPP_20) || JSON_HAS_THREE_WAY_COMPARISON
                     if ((i == 12 && j == 13) || (i == 13 && j == 12))
                     {
                         continue;