tests: Fix python line break in m5_exit test

tests: Fix python line break in m5_exit test

A bug has been introduced with the new test url.
The line break should have used a backslash or (this is the recommended
way by PEP8) the implied line continuation via parenthesis.
This error was preventing the test to be loaded with the error message:

Exception thrown while loading
"/tmpfs/src/git/jenkins-gem5-prod/tests/gem5/m5_util/test_exit.py"
Ignoring all tests in this file.

and was not producing a failure (the test was not run: it was jus
ignored).

Change-Id: I0afe252d66d2f6546caaf5e7be811f34f88df82c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24625
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>