more tests should run in parallel

[froydnj]

I had to run the entirety of test_other.py today due to my own mistakes in getting individual tests to run. Running this one test file was...not fun:

Ran 271 tests in 1970.620s

The only test suite where parallel test running is currently supported is test_core:

https://github.com/kripken/emscripten/blob/incoming/tests/runner.py#L1279

Surely other test suites could reasonably be converted to be run in parallel?

[kripken]

Yeah, other is slow...

The reason it isn't parallel is because it has a few tests which use a global location, the canonical temp dir. When EMCC_DEBUG=1 is in the env, we emit logging and write temp files to /tmp/emscripten_temp (by default), and a few tests depend on that.

We could fix this by adding a decorator for "this test must run all by itself, not in parallel with others", and that decorate would grab a lock, and we'd add that decorator to just the few tests that need it (just a grep for EMCC_DEBUG should be enough). And then adding other to the paralellizable tests suites should work.

[saschanaz]

(See also #5817)