[easylog][ut]add ut for easylog severity

[qicosmos]

Why

add ut for severity;

about #441

What is changing

Example

  CHECK(easylog::severity_str(easylog::Severity::WARN) ==
        easylog::severity_str(easylog::Severity::WARNING));
  CHECK(easylog::severity_str(easylog::Severity::CRITICAL) ==
        easylog::severity_str(easylog::Severity::FATAL));

[siriuslzx]

Why

add ut for severity;

about #441

What is changing

Example

  CHECK(easylog::severity_str(easylog::Severity::WARN) ==
        easylog::severity_str(easylog::Severity::WARNING));
  CHECK(easylog::severity_str(easylog::Severity::CRITICAL) ==
        easylog::severity_str(easylog::Severity::FATAL));

请问怎么测试 ANSI escape code，即测试WARN和WARNING的终端颜色一致。FATAL运行一次程序就退出了，又该怎么测试？

[qicosmos]

fatal 会退出，测warning 就行了。