Skip a flaky GC.compact test for YJIT

This test has failed without YJIT too:
https://rubyci.s3.amazonaws.com/debian11/ruby-master/log/20240301T063003Z.fail.html.gz

However, it somehow fails more often on "YJIT Ubuntu / check RUSTC='rustc +1.58.0'" job.

Nobody is actively looking into this issue, and it's most likely not a
YJIT-specific issue, so let me silence this false positive until we
start working on this.

test/ruby/test_symbol.rb

@@ -120,6 +120,7 @@ def test_inspect
 
   def test_inspect_under_gc_compact_stress
     omit "compaction doesn't work well on s390x" if RUBY_PLATFORM =~ /s390x/ # https://github.com/ruby/ruby/pull/5077
+    omit "very flaky on many platforms, more so with YJIT enabled" if defined?(RubyVM::YJIT) && RubyVM::YJIT.enabled?
     EnvUtil.under_gc_compact_stress do
       assert_inspect_evaled(':testing')
     end