Add a check that we've made it into the C runtime when doing dlopen().

This should give us a more informative failure mode than the one
mentioned in the README.
Relink tests when the library changes.

For the dynamically-linked tests, it wasn't a big deal because the
updated library was always picked up.  But for glx_static, we were
actually testing stale code.
Add a test showing a way that our ifuncs are broken.