Fix test runner --fail-fast test flake

Without the sleep, the test has high variability in the amount of test
ran before stopping. With the sleep, the test is pretty stable with
negligeable impact on run time.

railties/test/application/test_runner_test.rb

@@ -782,8 +782,9 @@ def test_verify_fail_fast
             assert false
           end
 
-          10.times do |n|
+          4.times do |n|
             define_method("test_verify_fail_fast_\#{n}") do
+              sleep 0.1
               assert true
             end
           end