patch 8.2.0615: regexp benchmark stest is old style

Problem:    Regexp benchmark stest is old style.
Solution:   Make it a new style test.  Fix using a NULL list.  Add more tests.
            (Yegappan Lakshmanan, closes #5963)
diff --git a/src/testdir/Make_dos.mak b/src/testdir/Make_dos.mak
index 66839d6..0e07ed5 100644
--- a/src/testdir/Make_dos.mak
+++ b/src/testdir/Make_dos.mak
@@ -107,12 +107,13 @@
 	-if exist test_result.log del test_result.log
 	-if exist messages del messages
 
-benchmark:
-	bench_re_freeze.out
+benchmark: test_bench_regexp.res
 
-bench_re_freeze.out: bench_re_freeze.vim
+test_bench_regexp.res: test_bench_regexp.vim
 	-if exist benchmark.out del benchmark.out
-	$(VIMPROG) -u dos.vim $(NO_INITS) $*.in
+	@echo $(VIMPROG) > vimcmd
+	$(VIMPROG) -u NONE $(NO_INITS) -S runtest.vim $*.vim
+	@del vimcmd
 	@IF EXIST benchmark.out ( type benchmark.out )
 
 # New style of tests uses Vim script with assert calls.  These are easier