UT: do not hide an original error in test resource ctxtmgr

UT: do not hide an original error in test resource ctxtmgr

In unit tests, resource contextmanagers such as network(), subnet()
try to delete themselves after returning from yield even if an
exception occurs. However when an exception occurs, there is a case
where deletion fails. In this case original exception will be hidden
and it makes difficult to debug test failures.

Before each test starts, resources like database entries will be
recreated, so there is no need to try to delete resources even
when an exception occurs. This commit removes try-finally clause
from resource contextmanagers to make original errors visible.

Closes-Bug: #1295887
Change-Id: Ia844d2aa2c9fc036e643068c5284f64798963ee3