Create a UnifiedDiffMatcher to make golden test failures easier to understand (#6897)

Right now I think everyone has the habit of doing an autoupdate then
using source control for a diff. This is offering an option of better
diff output from the test.

For example:

```
TEST: toolchain/driver/testdata/fail_flush_errors.carbon !
Ran 1 tests in 81 ms wall time, 8 ms across threads
testing/file_test/file_test_base.cpp:264: Failure
Value of: SplitOutput(test_file.actual_stderr)
Expected: matches elements with union diff
  Actual: { "fail_flush_errors.carbon:22:3: error: name `undeclared1` not found [NameNotFound]", "  undeclared1;", "  ^~~~~~~~~~~", "", "fail_flush_errors.carbon:31:3: error: `Core.String` implicitly referenced here, but package `Core` not found [CoreNotFound]", "  \"undec\\x6Cared2\";", "  ^~~~~~~~~~~~~~~~", "", "fail_flush_errors.carbon:35:3: error: name `undeclared2` not found [NameNotFound]", "  undeclared2;", "  ^~~~~~~~~~~", "", "fail_flush_errors.carbon:43:3: error: name `undeclared3` not found [NameNotFound]", "  undeclared3;", "  ^~~~~~~~~~~", "", "" }, union diff (- expected, + actual):
=== diff in expected elements 0 to 2:
+ fail_flush_errors.carbon:22:3: error: name `undeclared1` not found [NameNotFound]
    undeclared1;
    ^~~~~~~~~~~

=== diff in expected elements 4 to 9:
    "undec\x6Cared2";
    ^~~~~~~~~~~~~~~~

+ fail_flush_errors.carbon:35:3: error: name `undeclared2` not found [NameNotFound]
    undeclared2;
    ^~~~~~~~~~~

=== diff end

Stack trace:
  0x55e476d29efd: Carbon::Testing::FileTestCase::TestBody()
  0x55e476dbd1f2: testing::internal::HandleExceptionsInMethodIfSupported<>()
  0x55e476dbcf57: testing::Test::Run()
  0x55e476dbf0bf: testing::TestInfo::Run()
... Google Test internal frames ...


To test this file alone, run:
  bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/driver/testdata/fail_flush_errors.carbon

testing/file_test/file_test_base.cpp:277: Failure
Failed
Autoupdate would make changes to the file content. Run:
bazel run //toolchain/testing:file_test -- --autoupdate --file_tests=toolchain/driver/testdata/fail_flush_errors.carbon
Stack trace:
  0x55e476d2a5f0: Carbon::Testing::FileTestCase::TestBody()
  0x55e476dbd1f2: testing::internal::HandleExceptionsInMethodIfSupported<>()
  0x55e476dbcf57: testing::Test::Run()
  0x55e476dbf0bf: testing::TestInfo::Run()
... Google Test internal frames ...

[  FAILED  ] ToolchainFileTest.toolchain/driver/testdata/fail_flush_errors.carbon, where GetParam() = toolchain/driver/testdata/fail_flush_errors.carbon (93 ms)
```

Assisted-by: Google Antigravity with Gemini

testing/file_test/file_test_base.cpp

@@ -50,6 +50,7 @@
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/ThreadPool.h"
+#include "testing/base/unified_diff_matcher.h"
 #include "testing/file_test/autoupdate.h"
 #include "testing/file_test/run_test.h"
 #include "testing/file_test/test_file.h"
@@ -119,7 +120,6 @@ static auto SplitOutput(llvm::StringRef output)
   llvm::StringRef(output).split(lines, "\n");
   return llvm::SmallVector<std::string_view>(lines.begin(), lines.end());
 }
-
 // Verify that the success and `fail_` prefix use correspond. Separately handle
 // both cases for clearer test failures.
 static auto CompareFailPrefix(llvm::StringRef filename, bool success) -> void {
@@ -259,13 +259,9 @@ auto FileTestCase::TestBody() -> void {
 
   } else {
     EXPECT_THAT(SplitOutput(test_file.actual_stdout),
-                ElementsAreArray(test_file.expected_stdout))
-        << "Actual text:\n"
-        << test_file.actual_stdout;
+                ElementsAreArrayWithUnifiedDiff(test_file.expected_stdout));
     EXPECT_THAT(SplitOutput(test_file.actual_stderr),
-                ElementsAreArray(test_file.expected_stderr))
-        << "Actual text:\n"
-        << test_file.actual_stderr;
+                ElementsAreArrayWithUnifiedDiff(test_file.expected_stderr));
   }
 
   if (HasFailure()) {