tests: remove ugly /bin/sh wrapper around each perl-based test script

tests: remove ugly /bin/sh wrapper around each perl-based test script

* tests/check.mk (TESTS_ENVIRONMENT): Save and restore TMPDIR around
envvar-check, so that the few scripts that require $TMPDIR don't fail.
This is also good to let a user's default TMPDIR setting be used e.g.,
in the search for an 'other-partition'.
FIXME: this is pretty ugly.  maybe undo it and find a better way.
(TESTS_ENVIRONMENT): Invoke perl scripts with $(PERL), and use -T
if the script requires that.  Otherwise, use $(SHELL).

* tests/misc/md5sum-newline: Create a file whose name contains
a newline in Perl (resort to using "system", since open refuses).

Fix old brokenness exposed by this change:
* tests/du/files0-from: Correct test not to rely on stdin
being attached to a non-tty.
* tests/misc/sort (3g, 3h, 3i): Likewise: add explicit empty input file.
Avoid warnings about using qw()-around-commas.

* tests/rm/fail-eperm: Now that this test is run from a temporary
subdirectory, adjust the full name of the "rm" program we're going
to run.

Change #!/bin/sh to #!/usr/bin/perl, and factor out the few lines
of boilerplate code to invoke perl.  Do not "require 5.00x";
a configure-time Perl test handles that
* tests/dd/skip-seek:
* tests/misc/base64:
* tests/misc/basename:
* tests/misc/cut:
* tests/misc/date:
* tests/misc/dircolors:
* tests/misc/dirname:
* tests/misc/expand:
* tests/misc/expr:
* tests/misc/factor:
* tests/misc/fmt:
* tests/misc/fold:
* tests/misc/head:
* tests/misc/head-elide-tail:
* tests/misc/join:
* tests/misc/ls-misc:
* tests/misc/md5sum:
* tests/misc/md5sum-newline:
* tests/misc/mktemp:
* tests/misc/od:
* tests/misc/paste:
* tests/misc/pr:
* tests/misc/printf-cov:
* tests/misc/seq:
* tests/misc/sha1sum:
* tests/misc/sha1sum-vec:
* tests/misc/sha224sum:
* tests/misc/sha256sum:
* tests/misc/sha384sum:
* tests/misc/sha512sum:
* tests/misc/sort-merge:
* tests/misc/stat-printf:
* tests/misc/sum:
* tests/misc/tac:
* tests/misc/tail:
* tests/misc/test:
* tests/misc/test-diag:
* tests/misc/tr:
* tests/misc/tsort:
* tests/misc/tty-eof:
* tests/misc/unexpand:
* tests/misc/uniq:
* tests/misc/wc:
* tests/misc/wc-files0-from:
* tests/misc/xstrtol:
* tests/mv/i-1:
* tests/pr/pr-tests:
* tests/rm/empty-name:
* tests/rm/fail-eperm:
* tests/rm/unreadable: