tests: use "compare exp out", not "compare out exp"

Likewise, when an empty file is expected, use "compare /dev/null out",
not "compare out /dev/null". I.e., specify the expected/desired contents
via the first file name.  Prompted by a suggestion from Bruno Haible
in http://thread.gmane.org/gmane.comp.gnu.grep.bugs/4020/focus=29154

Run these commands:

    git grep -l -E 'compare [^ ]+ exp' \
      |xargs perl -pi -e 's/\b(compare) (\S+) (exp\S*)/$1 $3 $2/'
    git grep -l -E 'compare [^ ]+ /dev/null' \
      |xargs perl -pi -e 's,\b(compare) (\S+) (/dev/null),$1 $3 $2,'

[here, there were none of the latter]

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