torture: Make results-directory date format completion-friendly

torture: Make results-directory date format completion-friendly

The names of the per-test results directories are of the form
2019.11.29-20:42:19.  This works, but the ":" characters make
tab-based shell name completion a bit onerous because the user must
remember to include a quote character somewhere before the first ":".
This commit therefore changes the ":" characters to periods, as in
2019.12.01-20.48.01", which allows tab-based completion to work more
naturally.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>