bench: allow logging benchmarks to be order-independent

The global logging object instance is not re-created for each run, so when
multiple logging benchmarks are run, each one after the first one still has
the logging categories enabled from the previous ones.  This commit disables
all categories at the start of each benchmark.

src/bench/logging.cpp

@@ -9,6 +9,9 @@
 
 static void Logging(benchmark::Bench& bench, const std::vector<const char*>& extra_args, const std::function<void()>& log)
 {
+    // Reset any enabled logging categories from a previous benchmark run.
+    LogInstance().DisableCategory(BCLog::LogFlags::ALL);
+
     TestingSetup test_setup{
         CBaseChainParams::REGTEST,
         extra_args,