tests: Skip coverage tests in JS_GC_ZEAL mode

See https://bugzilla.mozilla.org/show_bug.cgi?id=1447906

This works around a SpiderMonkey bug by skipping some of the coverage
tests, which will fail if there are too many garbage collections.

Closes #130.
8 jobs for 130-skip-gc-zeal-tests in 35 minutes and 12 seconds (queued for 13 seconds)
Status Name Job ID Coverage
  Static Analysis
passed cppcheck #18626

00:01:39

passed cpplint #18627

00:02:04

passed eslint #18628

00:01:20

 
  Test
passed fedora #18629

00:02:48

 
  Build
passed fedora_clang #18630

00:04:21

 
  Thorough Tests
passed coverage #18632

00:24:11

passed no_profiler #18631

00:07:21

passed sanitizer_gcc #18633

00:03:43