This test runs the verifier on each function and matches the resulting
verifier error against the "error:" annotation.
Move the existing verifier test into filetests/verifier/ and use the new
syntex.
This makes it possible to refer to entities defined in the source file,
using the source names prefixed with $.
For example, $v20 refers to the value by that name in the sources, even
if it was renumbered to 'vx0' in the parsed file.
This trait serves as a shared interface for the different kinds of test
commands the 'cton-util test' understands.
Many tests produce output that is run through filecheck for validation.
Provide a simple run_filecheck() function to help with this.
Implement the 'test cat' sub-test which is probably the simplest
possible.
