patch 8.2.5031: cannot easily run the benchmarks

Problem:    Cannot easily run the benchmarks.
Solution:   Have "make benchmark" in the src directory work.
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index c54e060..c782da9 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -90,6 +90,10 @@
 	-rm -f valgrind.*
 	-rm -f asan.*
 
+# Delete the files produced by benchmarking, so they can run again.
+benchmarkclean:
+	rm -f $(SCRIPTS_BENCH)
+
 nolog:
 	-rm -f test.log test_result.log messages