A vanished scratch directory should say so, not just recover #1167

Problem

#1164 made a run survive its scratch directory going missing, which stops a
valid test file being reported as unsourceable. But it recovers silently.

#1137 is open precisely because that directory goes missing on CI and nobody
can say what removes it. Recovering without a word keeps it that way: the next
occurrence would leave no trace at all, which is strictly worse for diagnosis
than the misleading error it replaced.

Fix

Warn once per run, on stderr, naming the directory:

bashunit: the run scratch directory disappeared mid-run: /tmp/bashunit/run/OSX/PE39pOPW
bashunit: recreating it; please report this with the run log (#1137).

Once per run, not per file — a run that loses the directory early would
otherwise repeat it for every remaining file.