Reduce RemoteInvoke usage in tests (dotnet/corefx#41741)

Reduce RemoteInvoke usage in tests (dotnet/corefx#41741)

Many of our RemoteInvokes are there purely because of needing to change the current thread culture but on UAP that actually impacting the whole process.  Until that's addressed, we can at least avoid the RemoteInvoke on all platforms other than UAP. There are also some RemoteInvokes that can be avoided by just serializing all of the (small number of) tests in a test assembly. And there's some additional cleanup to be done as well around not explicitly returning an exit code unless required.

This helps measurably with test performance.  Consider the System.ComponentModel.Annotations test suite.  On my machine, prior to these changes, it took ~25 seconds; after these changes, it takes ~0.3. It helps even more when running against a checked runtime, where all of our test costs are magnified.  With a checked runtime, prior to these changes, it takes ~196 seconds; after these changes, it takes ~2.5 seconds.  This should help measurably with CI time in coreclr.

Commit migrated from https://github.com/dotnet/corefx/commit/cd18bafbabf7fe47601f6f790d157409034a2477