[PATCH] tests: use skip_ consistently.

[Karl Berry]

I belatedly noticed that I should have used skip_ instead of exit 77
in my new too-old-Emacs test. Plus found a wrong occurrence of just
"skip" without the _ in uninstall-fail.sh. Look ok?

(uninstall-fail has more failures in the case of a newer File::Path, due
to changes in rmtree as used in deltree.pl, but that'll be another day.)
--thanks, karl.

am-skip.diff
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