Fix tee statement for catching tempest output

The use of exec and tee doesn't seem to be quite right, and was
unreliable in terms of catching the exit status of the tempest
test as well as not catching the output when things went wrong.

This changes the way we do the redirect and the tee to something
that should be more robust and seems to work reliably in testing.

Change-Id: Ieb9d725839fb8e3f9e2e63a2b7b2e9c7c86713a2