tests: make target 'check-local' depend on 'atconfig'

I believe this may be related to sporadic failures we hit on Jenkins:

  test -z "atconfig" || rm -f atconfig
  ../../../tests/testsuite: line 987: ./atconfig: No such file or directory
  make[2]: *** [Makefile:1464: clean-local] Error 1

This dependency is present in all other Osmocom projects.

Change-Id: Id9bf06392ad31e1ecc66a5bd13069a915183a76b

tests/Makefile.am

@@ -244,7 +244,7 @@ python-tests: $(BUILT_SOURCES)
 	@echo "Not running python-based tests (determined at configure-time)"
 endif
 
-check-local: $(BUILT_SOURCES) $(TESTSUITE)
+check-local: atconfig $(BUILT_SOURCES) $(TESTSUITE)
 	$(SHELL) '$(TESTSUITE)' $(TESTSUITEFLAGS)
 	$(MAKE) $(AM_MAKEFLAGS) python-tests