Skip some timeout tests on s390x

They are too unstable on the machine.

```
  1) Failure:
TestRegexp#test_timeout_shorter_than_global [/home/chkbuild/chkbuild/tmp/build/20231018T230003Z/ruby/test/ruby/test_regexp.rb:1788]:
Expected |0.2 - 0.962938869| (0.7629388690000001) to be <= 0.15000000000000002.
```
https://rubyci.s3.amazonaws.com/s390x/ruby-master/log/20231018T230003Z.fail.html.gz

```
  1) Failure:
TestRegexp#test_timeout_longer_than_global [/home/chkbuild/chkbuild/tmp/build/20231017T140006Z/ruby/test/ruby/test_regexp.rb:1788]:
Expected |0.5 - 1.040696078| (0.5406960780000001) to be <= 0.375.
```
https://rubyci.s3.amazonaws.com/s390x/ruby-master/log/20231017T140006Z.fail.html.gz

1 files changed, 2 insertions, 0 deletions

diff --git a/test/ruby/test_regexp.rb b/test/ruby/test_regexp.rb
index 3a51ce357a..a0025b53ce 100644
--- a/test/ruby/test_regexp.rb
+++ b/test/ruby/test_regexp.rb
@@ -1790,10 +1790,12 @@ class TestRegexp < Test::Unit::TestCase
   end
 
   def test_timeout_shorter_than_global
+    omit "timeout test is too unstable on s390x" if RUBY_PLATFORM =~ /s390x/
     per_instance_redos_test(10, 0.2, 0.2)
   end
 
   def test_timeout_longer_than_global
+    omit "timeout test is too unstable on s390x" if RUBY_PLATFORM =~ /s390x/
     per_instance_redos_test(0.01, 0.5, 0.5)
   end