Add tests to user counters

[biojppm]

Now that user counters were integrated, tests should be added. During review of #262, #276 was suggested which added some tests. @EricWF Could you add your tests so that I pick up from there?

[EricWF]

I don't recall what I wanted to do. Feel free to proceed without me.

[biojppm]

ok.

[EricWF]

You should still add tests for user defined counters though.

[biojppm]

Exactly. Just realized I pushed the close button.

[biojppm]

@dominichamon @EricWF I finally got down and started doing this. It required adding facilities for getting at actual results from the benchmark. The commit above (6452883) allows the following usage:

void BM_Counters_Simple(benchmark::State& state) {
  while (state.KeepRunning()) {
  }
  state.counters["foo"] = 1;
  state.counters["bar"] = 2;
}
BENCHMARK(BM_Counters_Simple);
// make the results of BM_Counters_Simple be parsed, and call the given lambda.
CHECK_BENCHMARK_RESULTS("BM_Counters_Simple", [](ResultsCheckerEntry const& e) {
  // Check that "foo" read as an int is equal to 1
  // The EQ is appended to CHECK_ so it turns into a call to CHECK_EQ().
  CHECK_COUNTER_VALUE(e, int, "foo", EQ, 1);
  CHECK_COUNTER_VALUE(e, int, "bar", EQ, 2);
});

Are you OK with this approach? I would like to hear your thoughts on 6452883 before proceeding.

[EricWF]

The approach looks good to me. I'll need a couple days before I can look it over.

[biojppm]

Ok, I've gone ahead and completed it. Submitting a PR now.