t5516: refactor oddball tests

t5516: refactor oddball tests

t5516 sets up some utility functions for starting each test
with a clean slate. However, there were a few tests added
that do not use these functions, but instead make their own
repositories.

Let's bring these in line with the rest of the tests. Not
only do we reduce the number of lines, but these tests will
benefit from any further enhancements to the utility
scripts.

The conversion is pretty straightforward. Most of the tests
created a parent/child clone relationship, for which we now
use 'testrepo' as the parent.  One test looked in testrepo,
but relied on previous tests to have set it up; it now sets
up testrepo explicitly, which makes it a bit more robust to
changes in the script, as well.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>