Reimplemented coverage.py, using only gcov and with line+branch coverage

This also adds coverage support to the new test framework, which due to
reduction in scope, no longer needs aggregation and can be much
simpler. Really all we need to do is pass --coverage to GCC, which
builds its .gcda files during testing in a multi-process-safe manner.

The addition of branch coverage leverages information that was available
in both lcov and gcov.

This was made easier with the addition of the --json-format to gcov
in GCC 9.0, however the lax backwards compatibility for gcov's
intermediary options is a bit concerning. Hopefully --json-format
sticks around for a while.

scripts/test.py

@@ -985,11 +985,11 @@ if __name__ == "__main__":
     test_parser.add_argument('--gdb-main', action='store_true',
         help="Drop into gdb on test failure but stop at the beginning \
             of main.")
+    test_parser.add_argument('--exec', default=[], type=lambda e: e.split(),
+        help="Run under another executable.")
     test_parser.add_argument('--valgrind', action='store_true',
         help="Run under Valgrind to find memory errors. Implicitly sets \
             --isolate.")
-    test_parser.add_argument('--exec', default=[], type=lambda e: e.split(),
-        help="Run under another executable.")
     # compilation flags
     comp_parser = parser.add_argument_group('compilation options')
     comp_parser.add_argument('-c', '--compile', action='store_true',