Replace remaining OUnit tests by cram tests

Follow-on to #340.

[ ] Exclusions tests. Needs https://github.com/ocaml/dune/issues/3906; then update advanced.md.
[x] Multiple compilation unit/name mangling tests.
[x] Line number directive tests.
[x] .mli no-op processing tests.
[x] PPX interaction tests.
[x] Reporter tests.
[x] Thread-safety tests.
[x] Error message tests.
[x] Top-level usage tests.
[ ] It's also likely that the installation/usage/revdep tests can be converted to cram.

aantron / bisect_ppx