[6.2] swift-testing test fixes

[jmschonfeld]

• Explanation: Cherry-picks two fixes made to account for issues introduced by swift-testing's parallelism back to 6.2
• Scope: Mostly impacts unit test code only, with a small impact to JSON/PropertyList decoding when parsing numbers
• Original PRs: Prevent decimal_withFormat_localeDependent from relying on the current locale #1428, Update stringshims to always use clocale #1427
• Risk: Low - mostly impacts unit tests and non-unit test impact has good coverage with extensive local testing
• Testing: We've seen this failures stop appearing in mainline CI now that they've landed there for a few days, and we've done extensive local testing with these changes to ensure that the nondeterministic failures are eliminated
• Reviewers: @kperryua @itingliu

[jmschonfeld]

@swift-ci please test