Fix functional tests' '--capture' on Python 3

None of the commands' output is ever treated as binary, so we can just
always decode it as text.

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

system/lib.py

@@ -247,14 +247,14 @@ def prepare_fixture(self):
             for cmd in self.fixtureCmds:
                 output = self.run_cmd(cmd)
                 print("\n")
-                for line in output.decode("utf-8").split("\n"):
+                for line in output.split("\n"):
                     print(f"    {line}")
 
     def sort_lines(self, output):
         return "\n".join(sorted(self.ensure_utf8(output).split("\n")))
 
     def run(self):
-        output = self.run_cmd(self.runCmd, self.expectedCode).decode("utf-8")
+        output = self.run_cmd(self.runCmd, self.expectedCode)
         if self.sortOutput:
             output = self.sort_lines(output)
         self.output = self.output_processor(output)
@@ -288,6 +288,8 @@ def run_cmd(self, command, expected_code=0):
             proc = self._start_process(command, stdout=subprocess.PIPE)
             raw_output, _ = proc.communicate()
 
+            raw_output = raw_output.decode("utf-8")
+
             returncodes = [proc.returncode]
             is_aptly_command = False
             if isinstance(command, str):
@@ -299,12 +301,12 @@ def run_cmd(self, command, expected_code=0):
             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
-                match = re.search(r"EXIT: (\d)\n.*\n.*coverage: .*", raw_output.decode("utf-8"))
-                if match is None:
-                    raise Exception("no matches found in output '%s'" % raw_output.decode("utf-8"))
+                matches = re.findall(r"((.|\n)*)EXIT: (\d)\n.*\ncoverage: .*", raw_output)
+                if not matches:
+                    raise Exception("no matches found in output '%s'" % raw_output)
 
-                output = match.string[:match.start()].encode()
-                returncodes.append(int(match.group(1)))
+                output, _, returncode = matches[0]
+                returncodes.append(int(returncode))
             else:
                 output = raw_output
 
@@ -355,7 +357,7 @@ def check_output(self):
                 raise
 
     def check_cmd_output(self, command, gold_name, match_prepare=None, expected_code=0):
-        output = self.run_cmd(command, expected_code=expected_code).decode("utf-8")
+        output = self.run_cmd(command, expected_code=expected_code)
         try:
             self.verify_match(self.get_gold(gold_name), output, match_prepare)
         except:  # noqa: E722