Allow builder to run in test mode on untagged project

Particularly useful when creating new projects and wanting to avoid
creating an initial tag, this allows `tito build --test --srpm` etc
to run without any tag.
Merge branch 'test-without-tag' of https://github.com/domcleal/tito into domcleal-test-without-tag

Conflicts:
	test/functional/builder_tests.py