devel/libgit2: only build test suite when we request it and adjust tests

There's still one failure that seems to be occurring if you're running as
root [1] and I'm unable to work out where and/or if upstream ever fixed this test.

References:
https://github.com/archlinux/svntogit-packages/blob/packages/libgit2/trunk/PKGBUILD
1: https://github.com/libgit2/libgit2/issues/6307

PR: 268572