Store matching pattern in ProcessStarter.pattern_match

[dholth]

• fixes Save the line matching ProcessStarter.pattern #160

When I was doing this in my ProcessStarter subclass, I had to read lines in reverse to avoid finding previous runs in a persistent log file. May need to check against that here.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGELOG.rst, summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and make sure no tests failed.