Investigate why some tests in EditAndContinueStateMachineTests use resource strings

[JoeRobich]

In #82450 we added IsEnglishLocal to some tests. What we found is that some tests are using CodeAnalysisResource strings in their diagnostic verification:

• MissingIteratorStateMachineAttribute
• BadIteratorStateMachineAttribute
• SourceAsyncStateMachineAttribute
• NonIteratorToIterator_MissingAttribute

Others do not and have been marked with IsEnglishLocal:

• MissingAsyncStateMachineAttribute
• AddedAsyncStateMachineAttribute
• NonAsyncToAsync_MissingAttribute

Need to determine whether TestBase is doing the right thing

roslyn/src/Compilers/Test/Core/TestBase.cs

Lines 30 to 42 in 6fc483d

	// Force the UI culture to be the same as current culture. This will more fully exercise our
	// localization paths on machines that aren't en-US.
	//
	// Consider as an example our test infrastructure which runs machines in the es-ES locale. The
	// machine setup is such that CurrentCulture is es-ES but CurrentUICUlture is en-US. That means
	// our test infra isn't actually testing anything with respect to resource strings as they
	// all load with CurrentUICulture. This normalization means that when running on a machine
	// that doesn't use en-US we fully test our capabilities here.
	if (CultureInfo.CurrentUICulture != CultureInfo.CurrentCulture)
	{
	_originalUICulture = CultureInfo.CurrentUICulture;
	CultureInfo.CurrentUICulture = CultureInfo.CurrentCulture;
	}