Simplify assertKnownWidth to Verilog test (#3078)

Change the assertKnownWidth and assertInferredWidth test utility methods
to check Verilog directly as opposed to doing a full Verilator
compilation.  This dramatically improves the speed of these tests and
should help triage failures if there are problems.

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