[tests] Fix flakiness on some tests that assume object finalization

[tests] Fix flakiness on some tests that assume object finalization

A common pattern that we use across the test suite is to create objects on a new thread, join the thread, collect then expect certain objects to be dead/finalized since we assumed that thread to be no longer a source of pinning. This is not always the case since, after joining, the thread might still be alive and running unmanaged code related to the thread shutdown, which can reuse the stack that previously contained the managed references.

The workaround is to still allocate objects far down the stack, even if we are on a separate thread.

At the moment the helper can be used in tests which run under the TestDriver (since it's included in TestDriver.dll) and can also be included as a helper source for test bcl classes that need it.