[ruby/irb] Improve testing infra

(https://github.com/ruby/irb/pull/442)

* Add test_in_isolation task to run tests in isolation

This simulates how tests are run in Ruby CI and can detect some issues
before they're merged and break Ruby CI later.

* Run test_in_isolation task on CI

* Fix TestRaiseNoBacktraceException's setup

https://github.com/ruby/irb/commit/51f23c58b0

test/irb/test_raise_no_backtrace_exception.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: false
+require "tmpdir"
 
 require_relative "helper"