AssertionErrors should use process.stderr instead of console.log

Would be nice if I had the option to mute AssertionError traces by piping stderr elsewhere. I recommend a new reporter, maybe "default-pretty" and "default-verbose" which produce the same results but with the assertions errors going to stderr

该 issue 没有指明文件或测试。首先定位 reporter 的入口点以及当前针对 AssertionErrors 的 console.log 处理，然后运行现有的测试套件以了解输出行为。当提议的 reporter 行为在保留现有结果的同时允许通过 stderr 重定向断言跟踪信息时，即视为完成。

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