Add is_slow_test test decorator and EMTEST_SKIP_SLOW environment vari…

[sbc100]

…able

List of slow tests based on Jacob's list. I only included tests
that take more than 20 seconds.

Future work:

• Warn if test marked as slow runs in <10 seconds.
• Warn if test not marked as slow runs in >30 seconds.

[jgravelle-google]

s/IWTEST_/EMTEST_/ in the commit message

lgtm

[kripken]

Just curious, what's the motivation here?

[jgravelle-google]

Two parts to it:

1. Some of these tests are very slow. test_freetype takes ~2 minutes on my machine. Running 500 other not-slow tests takes ~2 minutes on my machine too. (this is with the wasm backend) So for local testing it can be easier to sanity-check with a faster subset of tests, and only run the full testsuite for submit-ready PRs.
2. For test scheduling, having a list of slow tests should let us start the longer tasks first more precisely than reverse-alphabetically sorting them, which should slightly speed up the full-testsuite case as well.

TODO: we should rename test_the_bullet to just test_bullet when we use this to affect test scheduling.

[kripken]

Yeah, we should sort the names in an optimal way.

If you just want a quick sanity check, you can do ./tests/runner.py random50 (or some other number). Or hand-pick e.g. binaryen2.test_c*.