Suspend many fibers test on JIT for now

https://github.com/ruby/ruby/runs/301411717

No C backtrace information and this is hard to fix immediately.
As CI doesn't provide helpful information, this should be debugged
locally or at least have more logs there.

Author: k0kubun

test/ruby/test_fiber.rb

@@ -34,6 +34,7 @@ def test_term
   end
 
   def test_many_fibers
+    skip 'This is unstable on GitHub Actions --jit-wait. TODO: debug it' if RubyVM::MJIT.enabled?
     max = 10_000
     assert_equal(max, max.times{
       Fiber.new{}