Add tests for Temporal.PlainMonthDay.prototype.toLocaleString

[Tomeshwari-02]

Description

This PR adds comprehensive test coverage for the existing toLocaleString() method on Temporal.PlainMonthDay as specified in issue #5085.

What's Included

Test Coverage (6 tests)

• ✅ Basic construction and property access
• ✅ Basic toLocaleString() functionality
• ✅ With calendar parameter ('iso8601')
• ✅ With locales parameter (single string and array)
• ✅ With options parameter
• ✅ Different month/day combinations (New Year, Independence Day, Halloween)

Implementation Details

The toLocaleString() method already exists in the codebase but had no test coverage. This PR adds:

1. Test file created: core/engine/src/builtins/temporal/plain_month_day/tests.rs
2. Module declaration added: #[cfg(test)] mod tests; in mod.rs
3. Comprehensive test suite: 6 test functions covering various scenarios

Current Implementation Note

The current implementation returns ISO string format (e.g., "12-25") until full ECMA-402 Intl.DateTimeFormat integration is completed. This approach:

• ✅ Matches the pattern used by other Temporal built-ins (PlainDate, PlainTime, ZonedDateTime)
• ✅ Is functional and usable
• ✅ Has clear TODO comments for future enhancement
• ✅ Follows Boa's incremental development approach

Testing Results

running 6 tests
test builtins::temporal::plain_month_day::tests::pmd_basic_construction ... ok
test builtins::temporal::plain_month_day::tests::pmd_to_locale_string_basic ... ok
test builtins::temporal::plain_month_day::tests::pmd_to_locale_string_with_calendar ... ok
test builtins::temporal::plain_month_day::tests::pmd_to_locale_string_with_locales ... ok
test builtins::temporal::plain_month_day::tests::pmd_to_locale_string_with_options ... ok
test builtins::temporal::plain_month_day::tests::pmd_to_locale_string_different_dates ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,073	0
Ignored	2,072	2,072	0
Failed	818	818	0
Panics	0	0	0
Conformance	94.54%	94.54%	0.00%

Tested main commit: 055ee0958ce332f3f99af27536a7c6f9a91def27
Tested PR commit: d9ea818078e18c5d51cfeeeb88e53c0b5aad4799
Compare commits: 055ee09...d9ea818