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.

openbsc/tests/Makefile.am

@@ -25,6 +25,7 @@ $(srcdir)/package.m4: $(top_srcdir)/configure.ac
 
 EXTRA_DIST = testsuite.at $(srcdir)/package.m4 $(TESTSUITE)
 TESTSUITE = $(srcdir)/testsuite
+DISTCLEANFILES = atconfig
 
 check-local: atconfig $(TESTSUITE)
 	$(SHELL) '$(TESTSUITE)' $(TESTSUITEFLAGS)
@@ -36,7 +37,6 @@ installcheck-local: atconfig $(TESTSUITE)
 clean-local:
 	test ! -f '$(TESTSUITE)' || \
 		$(SHELL) '$(TESTSUITE)' --clean
-	$(RM) -f atconfig
 
 AUTOM4TE = $(SHELL) $(top_srcdir)/missing --run autom4te
 AUTOTEST = $(AUTOM4TE) --language=autotest