tests: Don't delete atconfig in clean

This file is created in ./configure so we shouldn't remove it with make.
Otherwise ./configure && make clean && make check fails with:

make[3]: *** No rule to make target `atconfig', needed by `check-local'.
Stop.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index cd3337d..b60f675 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -79,6 +79,8 @@
              msgfile/msgfile_test.ok msgfile/msgconfig.cfg		\
              logging/logging_test.ok logging/logging_test.err
 
+DISTCLEANFILES = atconfig
+
 TESTSUITE = $(srcdir)/testsuite
 
 check-local: atconfig $(TESTSUITE)
@@ -91,7 +93,6 @@
 clean-local:
 	test ! -f '$(TESTSUITE)' || \
 		$(SHELL) '$(TESTSUITE)' --clean
-	$(RM) -f atconfig
 
 AUTOM4TE = $(SHELL) $(top_srcdir)/missing --run autom4te
 AUTOTEST = $(AUTOM4TE) --language=autotest