Revert "Skip test_race_exception on MJIT for now"

This reverts commit 7a75baa6e2.

Revert "Defer skip to avoid errors on ensure"

This reverts commit adfc8d6dba.

because 76e2370f13 is hoped to fix the
problem.

test/ruby/test_require.rb

@@ -507,7 +507,6 @@ class TestRequire < Test::Unit::TestCase
     path = nil
     stderr = $stderr
     verbose = $VERBOSE
-    skip 'require has had a race condition since 5931857281ce45c1c277aa86d1588119ab00a955' if RubyVM::MJIT.enabled?
     Tempfile.create(%w"bug5754 .rb") {|tmp|
       path = tmp.path
       tmp.print "#{<<~"begin;"}\n#{<<~"end;"}"