Revert "test runner should crash with non existing file argument."

This reverts commit 465f0fbca3.

This breaks some cases where non file / directory arguments are passed
to the runner (for example db:migrate).

I still think that we can get this to work. From what I can tell there
is no reason why db:migrate is passed along to `Minitest.run`. I'll
revert and investigate possible solutions.

railties/lib/rails/test_unit/test_requirer.rb

@@ -18,7 +18,7 @@ module Rails
             arg = arg.gsub(/:(\d+)?$/, '')
             if Dir.exist?(arg)
               "#{arg}/**/*_test.rb"
-            else
+            elsif File.file?(arg)
               arg
             end
           end

railties/test/application/test_runner_test.rb

@@ -307,11 +307,6 @@ module ApplicationTests
       end
     end
 
-    def test_crash_with_non_existing_file_or_dirname
-      error = capture(:stderr) { run_test_command('test/nope/niet_test.rb') }
-      assert_match(%r{cannot load such file.+test/nope/niet_test.rb}, error)
-    end
-
     def test_shows_filtered_backtrace_by_default
       create_backtrace_test