Run test suite from temporary test directory.

A couple of git tests were passing when the test suite was run from a
git checkout and failing when not. This was due to an underlying
errorneous assumption.

This commit fixes that assumption by making a new
git checkout, and also changes to the temporary directory setup for the
test at the start to hopefully prevent other tests from passing when
they shouldn't.

BUG:299579
FIXED-IN:1.15.1