refactor: address code review feedback

Makefile.am:
- Replace 4 hardcoded parallel commands with loop over FUNC_TEST_GROUPS
- Easy to extend by adding group numbers to the variable

functionaltests.c:
- Remove test counts from comments (prone to becoming outdated)
- Restore previous comment style with === separators for readability
- Replace switch with groups[] array structure for extensibility

proftest.h:
- Fix comment to describe current state, not the change made

CONTRIBUTING.md:
- Document test groups and parallel execution commands
- Add instructions for adding new test groups

CONTRIBUTING.md

@@ -147,9 +147,28 @@ The functional test suite uses [stabber](https://git.jabber.space/devs/stabber)
 Functional tests require stabber to be installed. Once installed, tests run as part of `make check`:
 
 ```bash
-make check
+make check                      # Run all tests (unit + functional)
+make check-functional-parallel  # Run functional tests in parallel (~3x faster)
+./tests/functionaltests/functionaltests    # Run all functional tests sequentially
+./tests/functionaltests/functionaltests 1  # Run specific group (1-4)
 ```
 
+#### Test groups
+
+Tests are organized into 4 groups for parallel execution:
+
+| Group | Description |
+|-------|-------------|
+| 1 | Connect, Ping, Rooms, Software |
+| 2 | Message, Receipts, Roster, Chat Session |
+| 3 | Presence, Disconnect |
+| 4 | MUC, Carbons |
+
+To add a new group:
+1. Define the test array in `functionaltests.c`
+2. Add entry to `groups[]` array
+3. Update `FUNC_TEST_GROUPS` in `Makefile.am`
+
 #### Writing functional tests
 
 Use content-based stubbing with stabber:
@@ -168,4 +187,5 @@ Guidelines:
 2. Use `stbbr_send(xml)` for presence, message, and other push-style stanzas.
 3. Keep assertions tolerant of ordering when possible; use `prof_output_regex()` for flexible matching.
 4. If timing issues appear, use `prof_timeout()` around critical expectations and reset afterwards.
+5. When adding new tests, place them in the appropriate group based on functionality.