fix(findcode): better finding of test code

This fix sorts the --paths to look for code.
Longest paths come first. This is for cases where
one checks out other sources within his source directory.
In such a case this structure can happen:
my_package
my_package/src/some_other_package

Now when my_package gets found first. It logs an import
error and then goes on.
by sorting with longest paths first. I always get the most
specific match first. Since the loop is short circuited here,
it will never look at my_package.

This fixes #15
Never mind, I see that options.prefix is already os.path.abspath'ed to the wazoo.

I see a similar for loop in `test_dirs()` elsewhere in this file.  Should it also process a sorted prefix list?

Perhaps the sorting ought to be done in `options.py`, where `options.prefix` is initialized?