Stderr in regular tests should probably go to out as well

[out] in regular tests should probably include stderr as well.

Recently, while making https://github.com/python/mypy/pull/19494, I learned that normal tests do not account for errors that go to stderr instead of stdout. This should be fixed, if possible (without performance penalty, so that normal tests can run a broader swath of tests.

This would allow more tests to be moved from slow cmdline tests to fast normal tests, à la https://github.com/python/mypy/issues/5966

I tried mucking about with the invocation of build.build in what struck me as the obvious ways, but even though I was able to get various ---------------------------- Captured stderr call ----------------------------- messages to pop off, I couldn't get the stderr to actually go into [out], even in a very crude way.

I'm not sure about how all this is plumbed, but it seems to me there's a minimal risk of erroneous stderr output messing up unrelated tests with false positives (or are they false negatives?), if we did have this working.

首先跟踪常规测试如何调用 build.build，以及 [out] 当前如何捕获输出，然后将该路径与所引用的慢速命令行测试进行比较。当常规测试在不产生性能损失的情况下将 stderr 包含在 [out] 中，从而能够将合适的测试移到更快的路径时，即表示完成。