Migrate swift-foundation to swift-testing

[jmschonfeld]

This PR contains the changes to migrate swift-foundation to swift-testing. All swift-foundation tests have been fully migrated to swift-testing with this change. A few notes about the changes here:

• The existing TestSupport module was removed and replaced with a re-purposed module to just contain a few utility helpers. It no longer contains expectEqual-like wrappers around XCTest and it no longer contains a list of type aliases
• FoundationEssentialsTests no longer links FoundationInternationalization. As a result, some tests needed to be moved from FoundationEssentialsTests to FoundationInternationalizationTests because they required having the i18n module loaded
• swift-testing runs tests in parallel by default, so some tests were updated to no longer rely on the current Locale/Calendar/TimeZone, and tests that did need to use these or other global state (like the CWD) were moved into nested serial suites (see FilePlaygroundTests as an example)
• We use the toolchain-provided version of swift-testing: this was done to avoid regressions to our test build time, and swift-foundation tests already require a pre-built toolchain with SwiftPM present so we are guaranteed to have a toolchain with swift-testing at that time