refactor(tests): improve functional test documentation and cleanup

- Add detailed documentation about test isolation in functionaltests.c
- Improve comments in proftest.c: buffer size explanation, stabber sync TODO
- Add reference to stabber issue #1 for stbbr_wait_stopped() API
- Document prof_output_regex() usage rationale in test_muc.c
- Enhance ping_server test comments for clarity
- Update CONTRIBUTING.md: fix function names (prof_output_regex, prof_timeout)
- Remove redundant commented-out test configurations from ci-build.sh
- Remove obsolete stabber recv suppressions from prof.supp (issue fixed)
- Add clear explanation for disabled assertion in test_chat_session.c
- Add comment explaining /connect max args in cmd_defs.c

src/command/cmd_defs.c

@@ -139,6 +139,7 @@ static const struct cmd_t command_defs[] = {
               "Show version and license information.")
     },
 
+    // Max args: account + server <s> + port <p> + tls <t> + auth <a> = 9
     { CMD_PREAMBLE("/connect",
                    parse_args, 0, 9, NULL)
       CMD_MAINFUNC(cmd_connect)