Update Plugin tests to treat copy_paths as a set

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Add tests for Plugin.add_copy_spec()/add_copy_specs()

Give add_copy_spec() and add_copy_specs() their own test cases.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Raise a TypeError if add_copy_specs() is called with a string

Since strings are iterable a plugin attempting to call
add_copy_specs("/something") results in a plugin calling
add_copy_spec("/"). Raise a TypeError if this happens.

Fixes Issue #141.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>