Test code restructured.

Summary:
To allow different providers to be tested independantly:
tests/general: provider agnostic tests
tests/imap: Imap tests
tests/data, tests/util: data + utility needed for tests.

Note: EAS tests live in the corresponding tests/eas directory in the inbox-eas repo.

Test Plan:
./runtests passes all as expected (i.e. depending on the value of the EASTEST flag)

Reviewers:
spang, emfree

CC:
emfree

Differential Revision: https://review.inboxapp.com/D63