tests: factor out the perl-requiring code in many test scripts

tests: factor out the perl-requiring code in many test scripts

* tests/require-perl: New file.
* tests/Makefile.am (EXTRA_DIST): Add require-perl.
* tests/dd/skip-seek: Use it, and remove manual tests.
* tests/du/files0-from: Likewise.
* tests/ls/nameless-uid: Likewise.
* tests/misc/base64: Likewise.
* tests/misc/basename: Likewise.
* tests/misc/cut: Likewise.
* tests/misc/date: Likewise.
* tests/misc/dircolors: Likewise.
* tests/misc/dirname: Likewise.
* tests/misc/expand: Likewise.
* tests/misc/expr: Likewise.
* tests/misc/factor: Likewise.
* tests/misc/fmt: Likewise.
* tests/misc/fold: Likewise.
* tests/misc/head-elide-tail: Likewise.
* tests/misc/ls-misc: Likewise.
* tests/misc/md5sum: Likewise.
* tests/misc/md5sum-newline: Likewise.
* tests/misc/mktemp: Likewise.
* tests/misc/od: Likewise.
* tests/misc/paste-no-nl: Likewise.
* tests/misc/pr: Likewise.
* tests/misc/pwd-long: Likewise.
* tests/misc/seq: Likewise.
* tests/misc/sha1sum: Likewise.
* tests/misc/sha1sum-vec: Likewise.
* tests/misc/sha224sum: Likewise.
* tests/misc/sha256sum: Likewise.
* tests/misc/sha384sum: Likewise.
* tests/misc/sha512sum: Likewise.
* tests/misc/sort-merge: Likewise.
* tests/misc/stat-printf: Likewise.
* tests/misc/sum: Likewise.
* tests/misc/sum-sysv: Likewise.
* tests/misc/test-diag: Likewise.
* tests/misc/tsort: Likewise.
* tests/misc/tty-eof: Likewise.
* tests/misc/unexpand: Likewise.
* tests/misc/wc-files0-from: Likewise.
* tests/misc/xstrtol: Likewise.
* tests/mv/i-1: Likewise.
* tests/rm/empty-name: Likewise.
* tests/rm/fail-eperm: Likewise.
* tests/rm/unreadable: Likewise.

(EXTRA_DIST): *do* require require-perl as a stand-alone, 'source'able script.