runners: bench: Added flags to control reading from bench probes

- -S/--probe         - Specify a probe to sample.
- -x/--probe-step    - Sample probes every n steps.
- --probe-runfreq    - Sample probes at this frequency in hz.
- -X/--probe-simfreq - Sample probes at this frequency in simulated hz.

Also:

- --trace-simfreq    - Sample trace output at this frequency in
                       simulated hz.

These give finer grain control over which probes we measure during
benching, and how we measure them.

These also introduce several exciting bench features:

- -S/--probe provides the ability to easily filter which probes you're
  interested in at runtime.

  This should replace the growing use of MASK defines in the benches.

- -x/--probe-step makes it easy to relax sampling rate when the amount
  of data overwhelms later scripts.

  This should replace the growing use of STEP defines in the benches.

- The additional concept of simfreq, which allows perf-esque sampling in
  simtime. This provides another option for intuitively relaxing probe
  sampling rate without sacrificing reproducibility.

  (runfreq depends on wall time, so good bye reproducibility, though may
  still be useful in interactive contexts.)

Note -S/--probe and -x/--probe-step replace MASK/STEP defines, which
have already proved their usefulness, but required reimplementation in
every bench case. An obvious contender to move into the bench_runner!

---

Note note that -S/--probe also supports some simple sample expressions,
allowing flexible step/simfreq/runfreq at the per-probe level:

- -Swrite=100    - Sample probe "write" every 100 steps
- -Swrite=100rhz - Sample probe "write" 100 times a runtime second
- -Swrite=100shz - Sample probe "write" 100 times a simulated second

Though I wonder how long it will take before I forget this feature
exists.

scripts/test.py

@@ -1179,7 +1179,7 @@ def run_stage(name, runner, test_ids, stdout_, trace_, output_, **args):
         last_id = None
         last_stdout = co.deque(maxlen=args.get('context', 5) + 1)
         last_assert = None
-        last_time = time.time()
+        last_runtime = time.time()
         try:
             while True:
                 # parse a line for state changes
@@ -1207,7 +1207,7 @@ def run_stage(name, runner, test_ids, stdout_, trace_, output_, **args):
                         last_id = m.group('id')
                         last_stdout.clear()
                         last_assert = None
-                        last_time = time.time()
+                        last_runtime = time.time()
                     elif op == 'powerloss':
                         last_id = m.group('id')
                         powerlosses += 1
@@ -1232,7 +1232,7 @@ def run_stage(name, runner, test_ids, stdout_, trace_, output_, **args):
                                     **defines,
                                     'test_passed': '1/1',
                                     'test_runtime': '%.6f' % (
-                                        time.time() - last_time)})
+                                        time.time() - last_runtime)})
                     elif op == 'skipped':
                         locals.seen_perms += 1
                     elif op == 'assert':