rustdoc: set panic output before starting compiler thread pool
When the compiler was updated to run on a thread pool, rustdoc's processing of compiler/doctest stderr/stdout was moved into each compiler thread. However, this caused output of the test to be lost if the test failed at *runtime* instead of compile time. This change sets up the `set_panic` call and output bomb before starting the compiler thread pool, so that the `Drop` call that writes back to the test's stdout happens after the test runs, not just after it compiles.
Fixes https://github.com/rust-lang/rust/issues/51162
487e961c6a