Add testing infrastructure for LFSC signatures (#4678)

Add testing infrastructure for LFSC signatures (#4678)

This commit adds testing infrastructure for LFSC signatures that is
enabled when CVC4 is configured with LFSC. The testing infrastructure
adopts run_test.py from https://github.com/CVC4/LFSC with minor
modifications (mainly adding support for a list of include directories
that are searched to resolve *.plf dependencies). The commit uses the
existing examples and test files from proofs/signatures as the initial
set of tests.

Co-authored-by: Alex Ozdemir aozdemir@hmc.edu