Enable BoringUtils Tests with MFC (#2981)

Re-enable BoringUtilsSpec and update it in minor ways to work with MFC.
The MLIR-based FIRRTL Compiler (MFC) now supports existing BoringUtils
APIs after:

- https://github.com/llvm/circt/commit/05ca6572e0a4962599cdb0f21cdc81dd9f4d86fb



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