Skip TestIRB::TestInit#test_recovery_sigint on Solaris

The test randomly gets stuck on Solaris:

http://rubyci.s3.amazonaws.com/solaris11-sunc/ruby-master/log/20211105T060014Z.fail.html.gz
```
  1) Error:
TestIRB::TestInit#test_recovery_sigint:
Timeout::Error: execution of assert_in_out_err expired timeout (100.0 sec)
pid 3195 killed by SIGTERM (signal 15)
| Switch to inspect mode.
| exit
|
```

I investigated the issue but I couldn't figure it out.
This random failure is noisy, which makes it difficult to track the CI.
So I skip the test on Soalris. Contribution is welcome.

test/irb/test_init.rb

@@ -62,6 +62,7 @@ def test_rc_file_in_subdir
     end
 
     def test_recovery_sigint
+      pend "This test gets stuck on Solaris for unknown reason; contribution is welcome" if RUBY_PLATFORM =~ /solaris/
       bundle_exec = ENV.key?('BUNDLE_GEMFILE') ? ['-rbundler/setup'] : []
       status = assert_in_out_err(bundle_exec + %w[-W0 -rirb -e binding.irb;loop{Process.kill("SIGINT",$$)} -- -f --], "exit\n", //, //)
       Process.kill("SIGKILL", status.pid) if !status.exited? && !status.stopped? && !status.signaled?