Summary: Now that D6685044 moved fastmanifest, we can also move the tests. Note
that test-fb-hgext-fastmanifest.t was moved seprately by Durham.
Test Plan: Ran all the tests.
Reviewers: quark, #mercurial, #sourcecontrol
Reviewed By: quark
Subscribers: durham, quark
Differential Revision: https://phabricator.intern.facebook.com/D6686584
Signature: 6686584:1515531785:3a9fa022f443faed6f3110eeef22d8ea72fcee3f
