Add warning a file given to Test contains no tests

[ChrisJefferson]

This adds a simple warning when a test file is run which doesn't contain any tests. This is most common because someone has misformatted their test file.

Also add some very simple tests for Test. None of these tests check for failures, because failures contain the full path of the failing test, which will change depending on where GAP is installed.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@544e716). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2524   +/-   ##
=========================================
  Coverage          ?   74.32%           
=========================================
  Files             ?      479           
  Lines             ?   238474           
  Branches          ?        0           
=========================================
  Hits              ?   177246           
  Misses            ?    61228           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/test.gi	65.48% <100%> (ø)

[fingolfin]

Looks good, but should be rebased to make sure there is no conflict with #2519

[markuspf]

Gna. Sorry I thought the tests had passed just now. I hope it didn't break :/