test clean-up: use env kill, not "$abs_top_builddir/src/kill"

[Jim Meyering]

I wrote "$abs_top_builddir/src/kill" to ensure that we'd run the
just-build coreutils binary, and not a shell built-in function by
the same name -- but that was back before I discovered
that "env builtin" is a much better way to do the job.

I changed some of these while fixing the pwd uses.
This fixes the kill-related ones:

0001-tests-use-env-kill-.-in-place-of-abs_top_builddir-sr.patch
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