Share all "Compiles" tests from VTableIndexStubGenerator with ComInterfaceGenerator

[jtschuster]

Adds all the "Compiled" tests for VTableStubGenerator to ComInterfaceGenerator tests.

Makes ICustomMarshallingSignatureTestProvider methods instance instead of static, requiring a TSignatureTestProvider to be passed to the contructor of Custom[Struct|Collection]MarshallingCodeSnippets. This lets us have different attributes in the snippets based on which generator the snippet is for (e.g. VTableIndexAttribute will be present for VTableStubGenerator, but not for ComInterfaceGenerator).

All tests pass in the ComInterfaceGenerator now, though we don't check for compiler errors if the generator produces warnings.

Includes the changes from #82740.

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Issue Details

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Adds all the "Compiled" tests for VTableStubGenerator to ComInterfaceGenerator tests.

Makes ICustomMarshallingSignatureTestProvider methods instance instead of static, requiring a TSignatureTestProvider to be passed to the contructor of Custom[Struct|Collection]MarshallingCodeSnippets. This lets us have different attributes in the snippets based on which generator the snippet is for (e.g. VTableIndexAttribute will be present for VTableStubGenerator, but not for ComInterfaceGenerator).

The tests that work with ComInterfaceGenerator are in the Shared...() snippets. The others are in ValidateComInterfaceSnippetsFailing, which passes if the testcase would fail in ValidateComInterfaceSnippets. This makes it easier to tell which tests were fixed by a change.

Author:	jtschuster
Assignees:	jtschuster
Labels:	area-System.Runtime.InteropServices
Milestone:	-

[jkoritzinsky]

LGTM other than a few nits