P0355R7_calendars_and_time_zones_io: Cleanup test coverage

[StephanTLavavej]

Followup to in-flight #6270; the first commit here will merge away harmlessly.

This is in response to a good Copilot code review suggestion; all of the edits (and regexes) are mine.

• Add want_value().
  • This makes it harder to forget to inspect the parsed value, and makes the structure of repetitive tests clearer.
  • Most, but not all, calls to test_parse() can be converted. The exceptions are when we need to parse abbrev/offset, or the assert has to do something unusual. As always, making the common cases simple helps the unusual cases stand out more.
• assert\((.+) && (.+)\); => assert($1); assert($2);
  • We try to avoid multi-asserts because they make the causes of failures harder to see.
• test_parse\((.+), (\w+)\);\n *assert\(\2 == (.+)\); => want_value($1, $2, $3);
  • Mechanically convert callsites.
• test_parse\((.+), (\w+)\); // (.+)\n *assert\(\2 == (.+)\); => want_value($1, $2, $4); // $3
  • Mechanically handle the callsites with comments on the test_parse() line. (The earlier commit correctly handled comments on the assert() line, and no callsites had comments on both lines.)
• Manually handle asserts that needed extra parens.
• Manually handle asserts that called .count(); we can instead construct values of the expected types.
• Manually add wanted values that weren't being inspected.
• Manually handle asserts that were batched up.
• Manually handle a confusing assert.
  • After parsing gt and verifying that it was equal to clock_cast<gps_clock>(ref), this parsed tt and then inspected it via clock_cast<gps_clock>, wanting it to be equal to gt. That's really confusing. Instead of performing the comparison as gps_clock, and involving the previously-parsed gt, let's go back to the original ref and view it through clock_cast<tai_clock>.
• Use source_location in want_value().
  • Copilot code review is working! 😹 Example after damaging the test:

    want_value() encountered an undesired value on line 261.
    Assertion failed: got_wanted, file D:\GitHub\STL\tests\std\tests\P0355R7_calendars_and_time_zones_io\test.cpp, line 203
    

[Copilot]

Pull request overview

This PR refactors and extends the <chrono> parsing IO tests for P0355R7_calendars_and_time_zones_io, primarily to make repetitive parse-and-compare assertions clearer and to add targeted coverage for modified-width parsing edge cases (follow-up to the in-flight work in #6270).

Changes:

• Introduces want_value() and converts many test_parse() + assert(value == expected) sequences to a single helper call; also splits combined asserts for clearer failure causes.
• Improves test diagnostics for unexpected parse success/failure by threading std::source_location into test_parse() / fail_parse().
• Adds parse_modified_maximum_characters() coverage and includes small bounds-check adjustments in the underlying parsing helpers (stl/inc/chrono, stl/inc/xloctime) to preserve room for null terminators.

Show a summary per file

File	Description
tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp	Refactors parse tests via want_value(), improves diagnostics with source_location, and adds modified-width parsing coverage.
stl/inc/xloctime	Adjusts digit-copy logic to preserve space for a null terminator (bounds safety).
stl/inc/chrono	Adjusts _Get_int() digit-copy bounds to preserve space for a null terminator (bounds safety).

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• Files reviewed: 3/3 changed files
• Comments generated: 1