Build tests with bsb and move them out of jscomp

[cknitt]

After #7063, the tests in jscomp/test were not compiled anymore.

This PR

• moves the tests from jscomp/test, jscomp/build_tests, jscomp/gentype_tests into a new top-level folder tests
• builds the tests that used to be compiled with ninja.js with the standard rescript/bsb tooling instead. (This also includes tests that were never compiled before as they were in subdirectories of jscomp/test.)
• removes the internal bsc option -make-runtime-test that is not needed anymore
• renames scripts/ciTest.js to scripts/test.js
• removes some obsolete files

[zth]

🥹

[zth]

Fantastic!