Improve the run-tests shell script

[ParthS007]

Later improvement: instead of having commands written twice, we could define shell functions like:

check_black () {
    black --check .
}

so that we can simply call check_black in appropriate places here...

In this way the commands to run will be defined in a single place (inside the shell function).

Originally posted by @tiborsimko in #36 (comment)