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- Remove test counts from comments (prone to becoming outdated)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

tests/functionaltests/proftest.h

@@ -2,9 +2,9 @@
 #define __H_PROFTEST
 
 /*
- * XDG paths are now dynamic, generated per-test based on stub_port.
- * This allows parallel test execution without file conflicts.
- * Use xdg_config_home and xdg_data_home variables instead of macros.
+ * XDG paths are dynamic and generated per-test based on stub_port.
+ * Each test instance uses unique directories (./tests/functionaltests/files/{port}/xdg_*)
+ * to allow parallel test execution without file conflicts.
  */
 extern char xdg_config_home[256];
 extern char xdg_data_home[256];