build: distcheck: do not leave a $TMPDIR/coreutils directory behind

build: distcheck: do not leave a $TMPDIR/coreutils directory behind

* dist-check.mk (tmpdir): Rename from TMPDIR.  Use ./tests/torture
unconditionally, rather than $TMPDIR-with-default-to-/tmp.
Otherwise, running "make distcheck" could leave an empty /tmp/coreutils
directory behind.
(tp): Simplify, now that it's always in the build-dir.
(taint-distcheck): Set HOME earlier, in case $(MAKE) misbehaves.
(my-instcheck, coreutils-path-check): Add diagnostics, so it's easier
to diagnose when each runs.
(coreutils-path-check): Run configure with --quiet, to reduce output.
Inspired by Ralf Wildenhues' report of /tmp/coreutils being left behind.