BF: fix imports I broke by moving the atom tests

When I moved the atom tests from the regreg/tests directory I didn't
realize that other tests were importing from them.  Fix these imports to
find the atom tests all_close routine.

Maybe atoms.tests.test_seminorms.all_close should be a utility function
somewhere not in tests?
Merge pull request #6 from matthew-brett/fix-test-imports

BF: fix imports I broke by moving the atom tests