patch 8.2.0621: after running tests asan files may remain

Problem:    After running tests asan files may remain.
Solution:   Clean up asan files with "make testclean".
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 92506e6..7f9ef04 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -100,6 +100,7 @@
 	-rm -f opt_test.vim test.log test_result.log messages
 	-rm -f $(RM_ON_RUN) $(RM_ON_START)
 	-rm -f valgrind.*
+	-rm -f asan.*
 
 test1.out: test1.in
 	-rm -rf $*.failed $(RM_ON_RUN) $(RM_ON_START) wrongtermsize