config: suites_dir and scenarios_dir are now a list of paths

This allows inheriting suites or scenarios from eg. sysmocom/ dir, while
still allowing to apply new suites and scenarios on top.

Change-Id: Icecdae32d400a6b6da2ebf167c1c795f7a74ae96
diff --git a/selftest/suite_test/paths.conf b/selftest/suite_test/paths.conf
index 2b0a274..a2f9124 100644
--- a/selftest/suite_test/paths.conf
+++ b/selftest/suite_test/paths.conf
@@ -1,2 +1,2 @@
 state_dir: ./test_work/state_dir
-suites_dir: .
+suites_dir: ['suitedirA', 'suitedirB']