Could you add tests for dlsym stubs?

Hi Nate. I've noticed that when I hardcode
ExecutionEngine::areDlsymStubsEnabled() to always return false,
check-lit still passes.

I'm working on fixing a couple bugs in the JIT that involve changing
how stubs are handled. I'm being careful around dlsym stubs, but I'm
not perfect and may make a mistake. I'd really appreciate if you could
add some tests for the code you added that would prevent me from
accidentally breaking it.