Fix deprecation test when ran multiple times

The deprecation warning is only printed on the first time the test is
ran, so subsequent runs will fail because the error message will not be
printed. This can be seen in the Ruby CI logs such as
http://ci.rvm.jp/results/trunk-repeat50@ruby-sp2-docker/4696313:

    1) Failure:
    TestIRB::RubyLexTest#test_initialising_the_old_top_level_ruby_lex [/tmp/ruby/src/trunk-repeat50/test/irb/test_ruby_lex.rb:210]:
    Expected /warning: constant ::RubyLex is deprecated/ to match "".

It can also be reproduced in the Ruby repo with the following test
command:

    make test/irb/test_ruby_lex.rb TESTOPTS=--repeat-count=2

This commit changes the test to use assert_in_out_err, which spawns a
new Ruby process.

test/irb/test_ruby_lex.rb

@@ -203,11 +203,10 @@ def test_assignment_expression_with_local_variable
     end
 
     def test_initialising_the_old_top_level_ruby_lex
-      _, err = capture_output do
+      assert_in_out_err(["--disable-gems", "-W:deprecated"], <<~RUBY, [], /warning: constant ::RubyLex is deprecated/)
+        require "irb"
         ::RubyLex.new(nil)
-      end
-
-      assert_match(/warning: constant ::RubyLex is deprecated/, err)
+      RUBY
     end
 
     private