tests: Reset umask to 022 while creating test repository

tests: Reset umask to 022 while creating test repository

In test-basic-root.sh we make assertions about the permissions
of files like baz/cow, which were created without an explicit chmod.
We can't do that unless we control the permissions.

For some reason the "debomatic" autobuilder used to do some Debian
archive rebuilds does the entire build including build-time tests
as uid 0 with umask 002, which broke those assertions. This seems
a weird thing to do, and I've opened a bug, but it also seems
reasonable to fix this test.

This also lets us remove a couple of existing workarounds for the
same issue.

Bug-Debian: https://bugs.debian.org/876138
Signed-off-by: Simon McVittie <smcv@collabora.com>
Closes: #1192
Approved by: cgwalters