compiletest: Capture panic messages via a custom panic hook
Currently, output-capture of panic messages relies on special cooperation between `#![feature(internal_output_capture)]` and the default panic hook. That's a problem if we want to perform our own output capture, because the default panic hook won't know about our custom output-capture mechanism.
We can work around that by installing a custom panic hook that prints equivalent panic messages to a buffer instead.
The custom hook is always installed, but delegates to the default panic hook unless a panic-capture buffer has been installed on the current thread. A panic-capture buffer is only installed on compiletest test threads (by the executor), and only if output-capture is enabled.
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Right now this PR doesn't provide any particular concrete benefits. But it will be essential as part of further efforts to replace compiletest's use of `#![feature(internal_output_capture)]` with our own output-capture mechanism.
r? jieyouxu
92bc467f36