zdtm: Make use of $test consistently

The $test variable was being used without being locally defined in
the start_test function. Define it locally for uniformity with other
functions and use it where convenient. Also make the definition in
case_error local for uniformity with other functions.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>