Some of the CI tests fail when using MMTk

[udesou]

Some Julia CI tests are currently incompatible with MMTk. They are related to:

• jl_n_gcthreads is set to 0 for MMTk since it uses its own worker threads. The tests in cmdlineargs.jl that are related to --gc-threads=X currently fail.
• MMTk does not support dumping a heap snapshot (at least not in the same format as the stock GC), some tests fail for this reason.
• When doing fastpath allocation we currently do not have a callback for the allocation profiler; some of the tests related to codegen and the allocation profiler fail when using MMTk.
• This test may fail, which I think is also because of how fastpath allocation works. (see also Failed test/checked: Evaluated: 0 ≈ 1 (rtol=0.9) mmtk/mmtk-julia#209)
• MMTk uses a different logging style than the stock GC. This test in misc.jl fails.
• --heap-size-hint is not implemented by MMTk since it only supports a hard heap limit. The tests related to that also fail.
• Some GC tests rely on the FULL_SWEEP_REASONS variable, which is specific to the stock GC. Furthermore, tests that check for page utilization also fail.

This issue is to document these tests until we decide whether they should be specific to the stock GC or until we implement the same semantics for MMTk (or change the semantics such the tests are compatible with both GCs).