Improve test output regex for better perf

The current regex runs in exponential time, which massively impacts the
runtime of the test suite, taking several seconds (~4s on my system)
just to perform a single match. By replacing the mix of re.findall + the
initial capture group with re.search + some string slicing, the time
spent matching the regex becomes nearly instant, e.g.:

    $ make system-test TESTS='Config*'

goes from taking ~10s to ~1.5s.

Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

system/lib.py

@@ -296,15 +296,12 @@ class BaseTest(object):
             if is_aptly_command:
                 # remove the last two rows as go tests always print PASS/FAIL and coverage in those
                 # two lines. This would otherwise fail the tests as they would not match gold
-                matches = re.findall(r"((.|\n)*)EXIT: (\d)\n.*\ncoverage: .*", raw_output.decode("utf-8"))
-                if not matches:
+                match = re.search(r"EXIT: (\d)\n.*\ncoverage: .*", raw_output.decode("utf-8"))
+                if match is None:
                     raise Exception("no matches found in output '%s'" % raw_output.decode("utf-8"))
 
-                output, _, returncode = matches[0]
-
-                output = output.encode()
-                returncodes.append(int(returncode))
-
+                output = match.string[:match.start()].encode()
+                returncodes.append(int(match.group(1)))
             else:
                 output = raw_output