This change defines manifest_dir and module_path expicitly in unit
tests so that modules installed under fixtures directory is properly
loaded.
Closes-Bug: #1930403
Change-Id: I50f23198d4bbcf17d741a589ad6dd2132d0a2e25
(cherry picked from commit 8063b23de1a3c835ab2d81864e2661717571ae9b)