Add support for testdox display names

[afbjorklund]

Make it possible to show test names as sentences:

https://bitfieldconsulting.com/golang/test-names

                                }
+                               if flags.testdox {
+                                       status.DisplayName = gotestdox.Prettify(status.TestName)
+                               }
                                allTests[key] = status

https://github.com/bitfield/gotestdox

• TestValidIsFalseForInvalidInput => Valid is false for invalid input

• TestValidIsTrueForValidInput => Valid is true for valid input