[#75804242] Consolidate documentation on how to run the tests

Make our documentation consistent so that we only document one way of running the tests.

Do this by replacing the Makefile with a stub bash script and update the Travis CI configuration accordingly.
I've renamed `common_setup_test.go` in 6f3c300 to `main_test.go`, which follows Go convention more closely but avoids the `undefined main.main` error seen when this was renamed to `main.go`. I've also avoided the use of the `main()` function, which was not being called by the test suite.