systemc: Fix a small bug in verify.py.

The src_dir method of the Test class was using the wrong attribute of
the test from the json file. It should use path, but was using src_dir.

Change-Id: Iaaaf31c31b11b68ecd7dd98807e6b78597cebb99
Reviewed-on: https://gem5-review.googlesource.com/12029
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>