bench: Add --sanity-check flag, use it in make check

The benchmarks are run as part of `make check` for a minimum sanity
check. The actual results are being ignored. So only run them for one
iteration.

This makes the `bench_bitcoin` part take 2m00 instead of 5m20 here.
Which is still too long (imo), but this needs to be solved in the
`WalletLoading*` benchmarks which take that long per iteration.

src/bench/bench.h

@@ -43,6 +43,7 @@ typedef std::function<void(Bench&)> BenchFunction;
 
 struct Args {
     bool is_list_only;
+    bool sanity_check;
     std::chrono::milliseconds min_time;
     std::vector<double> asymptote;
     fs::path output_csv;