Don't link any LLVM/MLIR library to the C API unit-test

The tests are intended to exercise the public C API and will link to a
specific shared library exposing only the C API, this library itself may
link to libMLIR.so.
If we link some LLVM library statically in the test themselves, we end
up with duplicated cl::opt registrations in LLVM. A possible setup if
these libraries were needed could be to link libMLIR.so directly when
available and link statically when it isn't available (in which case the
libary exposing the C API would be statically link and isolated from the
cl::opt registry, hopefully).

Differential Revision: https://reviews.llvm.org/D90993