Only run auto compact tests on platforms that support it

[tenderlove]

Some platforms (ppcle for example) can't support autocompact. Don't run
these tests on non-supported platforms

21a48d9 is causing failures on some CI machines (particularly the ones that don't support compaction). I'm not sure if this is the right way to guard against running the tests 😅

[eregon]

This is the corresponding check in CRuby tests: https://github.com/ruby/ruby/blob/3688b476710def7290e32656b200fefc538366d0/test/ruby/test_gc_compact.rb#L15-L25

[eregon]

Would it make sense for these methods to still succeed even on unsupported platforms?
For instance these methods are implemented on TruffleRuby but are just accessors (compaction is always enabled on JVM), similar to a few other GC methods:
https://github.com/oracle/truffleruby/blob/f2d856f50db7e3ac8447fe9edd54a4756b7b7436/src/main/ruby/truffleruby/core/gc.rb#L94-L132

What happens when running those tests on unsupported platforms?

[eregon]

I think ideally:

• Either these methods work (but have no effect) on unsupported platforms
• There is an easy way to test if supported, e.g. GC.respond_to?(:auto_compact=)/GC.auto_compact_supported?

It also might fine to simply call these methods and rescue a well known exception which means unsupported and use skip then.

I would much prefer not depending on a check needing Etc, GC::INTERNAL_CONSTANTS and other details which might change.

[eregon]

Found the failing log:
http://rubyci.s3.amazonaws.com/ppc64le/ruby-master/log/20211029T170005Z.fail.html.gz

1)
GC.auto_compact can set and get a boolean value ERROR
NotImplementedError: Automatic compaction isn't available on this platform
<internal:gc>:59:in `auto_compact='
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:15:in `block (3 levels) in <top (required)>'
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:4:in `block in <top (required)>'
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:3:in `<top (required)>'

2)
An exception occurred during: after :each
GC.auto_compact can set and get a boolean value ERROR
NotImplementedError: Automatic compaction isn't available on this platform
<internal:gc>:59:in `auto_compact='
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:10:in `block (3 levels) in <top (required)>'
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:4:in `block in <top (required)>'
/home/chkbuild/chkbuild/tmp/build/20211029T170005Z/ruby/spec/ruby/core/gc/auto_compact_spec.rb:3:in `<top (required)>'

I think it's best to just rescue NotImplementedError and skip those tests in that case then.
We can move the save/restore pattern inside the it to avoid errors in the after.

[eregon]

@tenderlove Thank you for the PR and making me aware of the issue.
I pushed a fix to CRuby (ruby/ruby@800dad6) and I'll commit it here to to fix rubyci.org.

[tenderlove]

@eregon thank you!!