[ORC-RT] Add two new jit-dlopen testcases, and switch to .c for dlopen tests.

We don't need precise control over the low-level behavior of these testcases so
C should be preferred for readability.

The new testcases test (1) the base dlopen case (running initializers and
deinitializers), and (2) the serial case of dlopen; dlclose; dlopen; dlclose,
where we expect the initializers and deinitializers to be run twice.