Convert ExtModuleImplSpec to use MFC (#2991)

Modify the ExtModuleImplSpec test to use the MFC and not the SFC.  This
includes one disabled test until firtool 1.30.0 is released.  There is a
silly bug where a supported Annotation (BlackBoxPathAnno) is supported
but rejected during parsing. See:

  - https://github.com/llvm/circt/commit/0285a98d96b8df898e02c5ed9528f869bff80dcf



Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>