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- Remove test counts from comments (prone to becoming outdated)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
- Add ARRAY_SIZE macro for cleaner code

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/functionaltests.c

@@ -38,7 +38,6 @@
 
 #include "config.h"
 
-#include "common.h"
 #include "proftest.h"
 #include "test_connect.h"
 #include "test_ping.h"
@@ -56,6 +55,8 @@
 /* Macro to wrap each test with setup/teardown functions */
 #define PROF_FUNC_TEST(test) cmocka_unit_test_setup_teardown(test, init_prof_test, close_prof_test)
 
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+
 int
 main(int argc, char* argv[])
 {