[Server][Test][loop-test] rename env name prefix MLPL_ -> HFL_
[CodingConvension] rename MLPL -> HFL
[Client][HowToTest] rename mlpl -> hfl
[Server][QUICK-REF] change LD_LIBRARY_PATH: mlpl -> hfl
[Server][HatoholArmPluginGate] rename variable prefix: mlpl -> hfl
[Server][Test] rename MLPL contains test function name

test_launchPluginProcessWithMLPLLoggerLevel(void)
test_launchPluginProcessWithHFL LoggerLevel(void)
                            ^^^^
[Server] rename mlpl -> hfl in comment
[Server][TODO] `mlpl::Logger` is now `hfl::Logger`.