Disable long-running fuzzer tests by default (#2887)

spirv-fuzz has useful tests that run the fuzzer and shrinker, to give
the whole tool a good shake up, effectively "fuzzing the fuzzer".  The
problems that this detects are sensitive to the source of randomness
that is used, which can change from test platform to test platform.
It is thus not a good idea to run these tests by default during
continuous integration - they may end up failing due to environtal
factors, making it look like an unrelated change has broken the fuzzer
when really the fuzzer has revealed an already-existing bug in itself.

This change makes the tests disabled by default; they can enabled
during dedicated testing of the fuzzer.