ttcn3/lib/testlib.py: implement the concept of test case groups

Change-Id: I687c221e4a6c7232290509cb11a3158d72b9c2c3
diff --git a/sysmocom/ttcn3/suites/ttcn3_bts_tests/scripts/BTS_Tests.cfg.tmpl b/sysmocom/ttcn3/suites/ttcn3_bts_tests/scripts/BTS_Tests.cfg.tmpl
index 510234e..4756ea1 100644
--- a/sysmocom/ttcn3/suites/ttcn3_bts_tests/scripts/BTS_Tests.cfg.tmpl
+++ b/sysmocom/ttcn3/suites/ttcn3_bts_tests/scripts/BTS_Tests.cfg.tmpl
@@ -29,4 +29,6 @@
 [MAIN_CONTROLLER]
 
 [EXECUTE]
-${ttcn3_test_execute}
+% for group in ttcn3_test_groups:
+${group}.control
+% endfor