bash_test: handle race condition.

Other tests may delete Bash files (e.g. `shcmd.sh`) while tests are
ongoing so ensure this doesn't cause a test failure.

Library/Homebrew/test/bash_test.rb

@@ -2,6 +2,7 @@ require "testing_env"
 
 class BashTests < Homebrew::TestCase
   def assert_valid_bash_syntax(file)
+    return unless file.exist?
     output = Utils.popen_read("/bin/bash -n #{file} 2>&1")
     assert $?.success?, output
   end