filter Foundry test contracts in the baseline runner

[mkzung]

Fixes #12.

The baseline runner drops files with test in the name, but that misses Foundry test contracts like Counter.t.sol (no test substring) and contracts under a test/ directory, so they get analyzed as sources and inflate the file count / findings.

I pulled the filter into a small is_test_file() helper that also catches *.t.sol and test//tests/ directories. It matches on the repo-relative path (so a data root whose own path contains test cannot filter every file), and it only ever excludes more files - nothing the old check already dropped gets re-included, so baselines can't silently grow.

$ python -m pytest tests/test_baseline_filter.py -q
12 passed