zdtm.py: Print test output on screen when possible

Right now zdtm spawns test into a pseudo-container and redirects its
output in a temp file. This is done not to mix outputs of tests, running
in parallel, with each other.

But if we run tests one by one or there's only one test in the list
we can safely print the output right on the screen.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>