add a verify script to test a single exercise

[glennj]

Personally, I find it convenient for exercise development.

Small changes to the verify_exercises script:

• remove the outdated comment block
• no need to explicitly test with -d, the glob pattern ending with / does that
• echo -n $'\n' is exactly identical to the plain echo
• let the exit code count the number of failed exercises.

[axtens]

Very good idea and addition. Thank you.