build: distcheck: add missing distclean files

Change-Id: I24d72b2b1bae52a1b2cf8a989396d2aac31d119e
diff --git a/src/tests/Makefile.am b/src/tests/Makefile.am
index 0fd050f..d36c018 100644
--- a/src/tests/Makefile.am
+++ b/src/tests/Makefile.am
@@ -42,6 +42,8 @@
 
 TESTSUITE = $(srcdir)/testsuite
 
+DISTCLEANFILES = atconfig
+
 check-local: atconfig $(TESTSUITE)
 	$(SHELL) '$(TESTSUITE)' $(TESTSUITEFLAGS)