Update sml-load-file and sml-load-test-file function, make it more robust.

To load a file and run sml repl -> Press C-c C-v inside a sml file.
To run the current files tests -> Press C-c C-r inside a sml file. It
assumes the tests are inside the same directory. For example, If your code file is hw1.sml your test file should be named as hw1tests.sml in the same directory