Create BCLog::Level::Trace log severity level

for verbose log messages for development or debugging only, as bitcoind may run
more slowly, that are more granular/frequent than the Debug log level, i.e. for
very high-frequency, low-level messages to be logged distinctly from
higher-level, less-frequent debug logging that could still be usable in production.

An example would be to log higher-level peer events (connection, disconnection,
misbehavior, eviction) as Debug, versus Trace for low-level, high-volume p2p
messages in the BCLog::NET category. This will enable the user to log only the
former without the latter, in order to focus on high-level peer management events.

With respect to the name, "trace" is suggested as the most granular level
in resources like the following:
- https://sematext.com/blog/logging-levels
- https://howtodoinjava.com/log4j2/logging-levels

Update the test framework and add test coverage.

src/test/i2p_tests.cpp

@@ -21,7 +21,7 @@ BOOST_FIXTURE_TEST_SUITE(i2p_tests, BasicTestingSetup)
 BOOST_AUTO_TEST_CASE(unlimited_recv)
 {
     const auto prev_log_level{LogInstance().LogLevel()};
-    LogInstance().SetLogLevel(BCLog::Level::Debug);
+    LogInstance().SetLogLevel(BCLog::Level::Trace);
     auto CreateSockOrig = CreateSock;
 
     // Mock CreateSock() to create MockSock.