test suite: t-check-only-bpd: Check $tmp/.. not ..

test suite: t-check-only-bpd: Check $tmp/.. not ..

The test might call t-ok in $p, say, or in $tmp.

The effect of this bug was that in tests where t-ok was called in
$tmp, it looked in $tmp/.. which might contain anything, rather than
$tmp as intended.  That means the test was not effective.

Also it causes a spurious failure of t-import-nonnative in the Ubuntu
CI infrastructure, where $AUTOPKGTEST_TMP/../autopkgtest-satdep.deb
happens to exist.

Reported-by: Mattia Rizzolo <mattia@debian.org>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>