Building utility function to check existence and content of user-written files.

[gvwilson]

• test_file_compare uses shell commands to check existence of user-written file and compare it against reference file.
• It expands environment variables: $HOME is standard, but it also assumes $ANSWERS has been set to the root directory containing solutions.
• It uses assert to test for the existence of the reference file: should this be done in some other way?
• It continues testing even if the user file doesn't exist: what is the correct way to short-cut this?

[machow]

@gvwilson I'll try to take a look at all your PRs today.

Also, sorry, I should create a development doc somewhere on DataCamp for python version requirements. Most projects needs to be compatible with python 3.5, since that's what we're running currently. The literal sting interpolation syntax (f"") was introduced in python 3.6, so the tests fail on Travis CI.

edit: note that I can quickly change it to python 3.5 compatible syntax when I look today.

[gvwilson]

@machow thanks, and not urgent - at this point I doubt we'll be able to teach shell scripts in V1 of this lesson, so I'm preparing a second branch that has them writing loops in the command line instead. If that flies, they'll be generating all their files with > redirects, which means we can trust cmp to give an accurate answer.