Add a test function to test Evoral::coverage() with all possible overlap
types. The first test (line 161) that expects OverlapExternal will fail
with the current implementation of coverage().
There's possibly still a discussion to be had about what the overlap type of
ranges with negative lengths should be: there are currently places in the main
Ardour code base where coverage() is called with ranges where start > end.
4358b8db79