system tests: use repository mirrors on S3 for reproducibility

no direct internet download from apt repositories,
which over time will change or cease to exist.

also migrate to gpg2 on newer ubuntu.

system/lib.py

@@ -251,7 +251,7 @@ class BaseTest(object):
         return "\n".join(sorted(self.ensure_utf8(output).split("\n")))
 
     def run(self):
-        output = self.run_cmd(self.runCmd, self.expectedCode)
+        output = self.run_cmd(self.runCmd, self.expectedCode).decode("utf-8")
         if self.sortOutput:
             output = self.sort_lines(output)
         self.output = self.output_processor(output)
@@ -342,7 +342,7 @@ class BaseTest(object):
         try:
             self.verify_match(self.get_gold(), self.output,
                               match_prepare=self.outputMatchPrepare)
-        except:  # noqa: E722
+        except Exception:  # noqa: E722
             if self.captureResults:
                 if self.outputMatchPrepare is not None:
                     self.output = self.outputMatchPrepare(self.output)
@@ -352,7 +352,7 @@ class BaseTest(object):
                 raise
 
     def check_cmd_output(self, command, gold_name, match_prepare=None, expected_code=0):
-        output = self.run_cmd(command, expected_code=expected_code)
+        output = self.run_cmd(command, expected_code=expected_code).decode("utf-8")
         try:
             self.verify_match(self.get_gold(gold_name), output, match_prepare)
         except:  # noqa: E722
@@ -439,7 +439,7 @@ class BaseTest(object):
                 diff += "wrong value '%s' for key '%s', expected '%s'\n" % (
                     v, k, b[k])
         if diff:
-            raise Exception("content doesn't match:\n" + diff)
+            raise Exception("content subset doesn't match:\n" + diff)
 
     def ensure_utf8(self, a):
         if isinstance(a, bytes):