one test file per exercise => let user specify a combination of tests

[adrienjoly]

idea: we could use "Argument passing and parsing" from https://www.twilio.com/blog/npm-scripts to pass the combination of tests

=> something like:

$ TESTS="1.1 1.2 1.3 1.4 1.5" ./eval-student-submissions.sh ./submissions/*

...instead of:

$ TESTER=test-ex-1-5.js ./eval-student-submissions.sh ./submissions/*

[adrienjoly]

It's actually possible to run TESTER=test-ex-1-*.js ./eval-student-submissions.sh ./submissions/*:-)

In order for Ava to respect the order of test files, add the --serial flag.