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)

src/testdir/test_bench_regexp.vim

@@ -0,0 +1,24 @@
+" Test for benchmarking the RE engine
+
+source check.vim
+CheckFeature reltime
+
+func Measure(file, pattern, arg)
+  for re in range(3)
+    let sstart = reltime()
+    let before = ['set re=' .. re]
+    let after = ['call search("' .. escape(a:pattern, '\\') .. '", "", "", 10000)']
+    let after += ['quit!']
+    let args = empty(a:arg) ? '' : a:arg .. ' ' .. a:file
+    call RunVim(before, after, args)
+    let s = 'file: ' .. a:file .. ', re: ' .. re ..
+          \ ', time: ' .. reltimestr(reltime(sstart))
+    call writefile([s], 'benchmark.out', "a")
+  endfor
+endfunc
+
+func Test_Regex_Benchmark()
+  call Measure('samples/re.freeze.txt', '\s\+\%#\@<!$', '+5')
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab