fix: evaluate SkipAttribute before data source initialization

[thomhurst]

Summary

Fixes #4737

• Reorder registration phase: Fire OnTestRegistered event receivers (including SkipAttribute) before RegisterTestArgumentsAsync in TestFilterService.RegisterTest, and skip argument registration entirely when SkipReason is set. This prevents ClassDataSource initialization for tests that should be skipped.
• Early skip check in execution: Check SkipReason in ExecuteTestInternalAsync before entering retry/timeout logic and instance creation, avoiding unnecessary work for already-skipped tests.
• Pre-instance skip check: Check SkipReason in ExecuteTestLifecycleAsync before CreateInstanceAsync() to prevent data source IAsyncInitializer calls that would fail or waste resources for skipped tests.

Root cause

Derived SkipAttribute subclasses (e.g., CI-environment skip attributes) evaluate ShouldSkip() at runtime during OnTestRegistered. Previously, RegisterTestArgumentsAsync ran before event receivers, and CreateInstanceAsync() (which triggers IAsyncInitializer.InitializeAsync() on data sources) ran before the SkipReason check. If initialization threw (e.g., database unavailable in CI), the exception escaped before the skip check could run, marking the test as failed instead of skipped.

Test plan

• New regression test: derived SkipAttribute + ClassDataSource with failing IAsyncInitializer → test is skipped, not failed
• New regression test: derived SkipAttribute + ClassDataSource with succeeding initializer → test is skipped
• Both tests run in all 3 modes (SourceGenerated, Reflection, AOT)
• Existing skip tests pass: SkipTests, DynamicSkipReasonTests, SkipInHooksTests, SkipTestExceptionTests
• Existing data source tests pass: DataSourceCountTests, DataSourceExceptionPropagationTests, GenericMethodWithDataSourceTests, PropertyInjectionInitFailureTests

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