* Added failing test for source with trailing slash
This test is nearly identical to the one above it, with the only
change being the source that is used. This may end up turning into
a fixture instead if the tests end up being identical after the
fix is made.
* Strip trailing slash for relative source paths
This fixes an issue introduced in 45cf7936ee605cfe06f7f5967a72a73198960120
where using `relative_files=True` and `src` with a trailing slash,
the source inserted as `<sources>` in the XML report would also have
a trailing slash. This also fixes an issue introduced in the same
commit where an empty `<source>` would be inserted as well for cases
where the `src` has a trailing slash.