Add tests for OAVariantLib

[huoyaoyuan]

Test for #100176.

OAVariantLib.ChangeType will be called when a value is passed from native COM to managed via IDispatch (parameter in ComToClr call, or return value of ClrToCom call, or out value from byref parameter), and the type mismatches from managed signature. The calling order of current behavior is roughly:

• OleVariant::MarshalObjectForOleVariant, marshals a default managed representation of the VARIANT. Invalid/unsupported VARIANT type will be rejected here and throws InvalidOleVariantTypeException.
• RuntimeType.ForwardCallToInvokeMember, OleAutBinder.ChangeType in managed code, handles common cases like type already matches, primitive->enum and ref non-primitive->underlying type. It misses the case of ref primitive->underlying type and will always treat it as a type mismatch.
• OAVariantLib.ChangeType and OAVariant_ChangeType: Convert source value and target type back to VARIANT and VARENUM. Specially handles the case when target is System.Color. Uses a custom mapping logic that's slightly different. Types rejected here will throw NotSupportedException, caught by OleAutBinder.ChangeType and rethrown as COMException(0x80020005).
• VariantChangeTypeEx. It does the conversion with its own order. Errors will be thrown as-is, typically InvalidCastException.

This PR adds test for:

• Supported conversions for different positions, including parameter in ComToClr call and return value of ClrToCom call.
• Different rejected behaviors at each path, only for the return value case.

The test is not exhaustive and try not to exercise of detailed behavior of VariantChangeTypeEx.

@AaronRobinsonMSFT @jkotas