t/txinfo-vtexi4.sh and timezone failure

[Karl Berry]

The automake test t/txinfo-vtexi4.sh failed when localtime and UTC were
on different days, resulting in the GREPDATE for the @UPDATED@ string
not matching (among possible others).

mdate-sh always uses TZ=UTC0. This tiny change makes the test do the
same thing.

I wondered if there were other tests that might have the same problem,
e.g., if TZ would be better set in test-init.sh (although granted this
was the only one failing). However, install-sh-option-C.sh is the only
other test that was setting TZ, and I didn't see others invoking date,
either. So Jim and I figured just patching vtexi4 is reasonable.

Unrelated but so trivial did not at all seem worth separating: the doc
in vtexi4.sh referred to itself when it didn't mean to.

Any comments or suggestions before I attempt to push this? --thanks, karl.

txi-test-tz.diff
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