Follow-on to #340.

• Exclusions tests. Needs Passing arguments to instrumentation backends ocaml/dune#3906; then update advanced.md.
• Multiple compilation unit/name mangling tests.
• Line number directive tests.
• .mli no-op processing tests.
• PPX interaction tests.
• Reporter tests.
• Thread-safety tests.
• Error message tests.
• Top-level usage tests.
• It's also likely that the installation/usage/revdep tests can be converted to cram.