resourceloader: Improve test coverage of ResourceLoader::register()

resourceloader: Improve test coverage of ResourceLoader::register()

Verify that calling register() twice does not throw, but warns,
and that the last registration wins.

This behaviour was actually surprising to me because it used to
throw, and I'd assume that when we added the warning, the behaviour
would go from fatal to non-fatal, but keep that the last one is
at fault/unsupported.

Perhaps b1ea0612 / d3e3bcfd6 (T116628) should've added a return
statement. Oh well, we can consider changing that later, but at
least test for it.

Change-Id: I955132868146ea5bf88c9b9e648c84d8196cb1f9