[Test] Cleaned up how a bunch of tests were ignored

Previously, many tests just had a return statement when the test shouldn't be executed (e.g. Unix-only).
This made it look like the test was successful when in fact it wasn't executed at all.
Assert.Ignore() is now used instead to correctly exclude those tests.

Changes released under MIT/X11.

I know this looks like a large changeset, but it is effectively a no-op.
Let me know if you prefer multiple commits or anything else :smile: