Alcotezt 🤫: make lib_store/unix/test's output depend on Tezt log level

Context

Currently, lib_store/unix tests log wildly to stdout, creating noise in tezt logs.

This MR creates an Internal_event.SINK that logs through Tezt and that maps Internal_event log levels to Tezt log levels. Then I use it in lib_store.

Discussion

Not sure where to put tezt_sink.ml because I want to use it elsewhere :/ For now, put it with the test.

Manually testing the MR

dune exec src/lib_store/unix/test/main.exe -- -f src/lib_store/unix/test/test_store.ml --only 1 # no output
dune exec src/lib_store/unix/test/main.exe -- -f src/lib_store/unix/test/test_store.ml --only 1 --verbose # lots of output
dune exec src/lib_store/unix/test/main.exe -- -f src/lib_store/unix/test/test_store.ml --only 1 --log-file tezt.log # check that tezt.log contains the verbose output

Checklist

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