tests: Fix syntax error in cpu_tests/test.py

tests: Fix syntax error in cpu_tests/test.py

The testsuite was not loaded with the following error:

Exception thrown while loading
<gem5>/tests/gem5/cpu_tests/test.py

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: I1e88b8957bb24471e1bb6113ffc7c78886b6ed70
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39295
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>