bench: explicitly make all current benchmarks "high" priority

no-functional changes. Only have set the priority level explicitly
on every BENCHMARK macro call.

src/bench/mempool_stress.cpp

@@ -114,5 +114,5 @@ static void MempoolCheck(benchmark::Bench& bench)
     });
 }
 
-BENCHMARK(ComplexMemPool);
-BENCHMARK(MempoolCheck);
+BENCHMARK(ComplexMemPool, benchmark::PriorityLevel::HIGH);
+BENCHMARK(MempoolCheck, benchmark::PriorityLevel::HIGH);