Add unit tests for RCTInstance eager main-queue module setup

[javache]

Summary:
There was no unit-test coverage for enableEagerMainQueueModulesOnIOS, the
runtime gate that lets selected native modules initialize on the main thread
during React Native init. These characterization tests pin down the behavior
end-to-end through the real production path:

• With the flag off, RCTInstanceDelegate.unstableModulesRequiringMainQueueSetup
  is never consulted and no eager module is instantiated.
• With the flag on, the delegate is consulted exactly once and each returned
  module name flows through RCTTurboModuleManager to a real -init on the
  main queue.
• The JS thread that drives _loadScriptFromSource: blocks at the beforeLoad
  hook until the eager module setup completes. The bundle-load test runs init
  on a background queue, gates FakeEagerModule.init on a semaphore, and
  observes the RCTInstanceDidLoadBundle notification fired from afterLoad —
  asserting that by the time the gate opens, the module's constructor has fully
  returned.

The tests exercise the real RCTModuleRegistry + RCTTurboModuleManager path
(no subclass shims, no KVC into private ivars). The fixture provides a minimal
FakeEagerModule conforming to <RCTBridgeModule, RCTTurboModule> so it
survives TMM's _shouldCreateObjCModule: filter when interop is off.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D105953816

[meta-codesync]

@javache has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105953816.

[react-native-bot]

This pull request was successfully merged by @javache in 5e0fd8e

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[meta-codesync]

This pull request has been merged in 5e0fd8e.