vim-patch:8.2.0615: regexp benchmark stest is old style (#20940)

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 vim/vim#5963)

ad48e6c159

N/A patches:
vim-patch:9.0.0829: wrong counts in macro comment

test/benchmark/bench_regexp_spec.lua

@@ -0,0 +1,64 @@
+-- Test for benchmarking the RE engine.
+
+local helpers = require('test.functional.helpers')(after_each)
+local insert, source = helpers.insert, helpers.source
+local clear, command = helpers.clear, helpers.command
+
+-- Temporary file for gathering benchmarking results for each regexp engine.
+local result_file = 'benchmark.out'
+-- Fixture containing an HTML fragment that can make a search appear to freeze.
+local sample_file = 'src/nvim/testdir/samples/re.freeze.txt'
+
+-- Vim script code that does both the work and the benchmarking of that work.
+local measure_cmd =
+    [[call Measure(%d, ']] .. sample_file .. [[', '\s\+\%%#\@<!$', '+5')]]
+local measure_script = [[
+    func Measure(re, file, pattern, arg)
+      let sstart = reltime()
+
+      execute 'set re=' .. a:re
+      execute 'split' a:arg a:file
+      call search(a:pattern, '', '', 10000)
+      quit!
+
+      $put =printf('file: %s, re: %d, time: %s', a:file, a:re, reltimestr(reltime(sstart)))
+    endfunc]]
+
+describe('regexp search', function()
+  -- The test cases rely on a temporary result file, which we prepare and write
+  -- to disk.
+  setup(function()
+    clear()
+    source(measure_script)
+    insert('" Benchmark_results:')
+    command('write! ' .. result_file)
+  end)
+
+  -- At the end of the test run we just print the contents of the result file
+  -- for human inspection and promptly delete the file.
+  teardown(function()
+    print ''
+    for line in io.lines(result_file) do
+      print(line)
+    end
+    os.remove(result_file)
+  end)
+
+  it('is working with regexpengine=0', function()
+    local regexpengine = 0
+    command(string.format(measure_cmd, regexpengine))
+    command('write')
+  end)
+
+  it('is working with regexpengine=1', function()
+    local regexpengine = 1
+    command(string.format(measure_cmd, regexpengine))
+    command('write')
+  end)
+
+  it('is working with regexpengine=2', function()
+    local regexpengine = 2
+    command(string.format(measure_cmd, regexpengine))
+    command('write')
+  end)
+end)