Add option to install extra packages

withmock and mocktest now have a -P option which allows providing a
filename to specify a file listing extra packages to install into the
generated GOPATH.
Use -P to fix with_deps scenario

Now we can install extra packages into the generated GOPATH, use that
to fix the with_deps scenario.

Also, test each scenario with withmock and mocktest.
Improve parsing of package list

Don't add empty strings to the list of packages to test.

Also, if we don't find any packages to test, then give up.
Add some more scenarios