Clean up testApp and testAppConfig

Previously, testAppConfig added some extra files to the mock filesystem
before passing them to testConfig. However, those files are only needed
for TestApp so this converts TestApp to use test fixtures and populates
the mock filesystem with the extra files.

Without those extra files testApp and testAppConfig are equivalent to
testJava and testConfig respectively.

Bug: 181070625
Test: m nothing
Change-Id: I1d8a51223adde5b656948725bc4bc91040533625