test runner should crash with non existing file argument.

Before this patch, using `bin/rails test` with a non existing
file or directory argument would silently swallow the argument and
run the whole test suite.

After the patch the command fails with `cannot load such file --`.

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"
-            elsif File.file?(arg)
+            else
               arg
             end
           end

railties/test/application/test_runner_test.rb

@@ -307,6 +307,11 @@ 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