Auto merge of rust-lang#103180 - ferrocene:pa-qemu-user-mode, r=Mark-Simulacrum

Handle core dumps output in QEMU user mode

In addition to the whole-system emulation/virtualization, QEMU also supports user-mode emulation, where the emulation happens as a normal process inside the parent system. This allows running most tests by simply spawning remote-test-server inside user-mode emulation.

Unfortunately, QEMU always writes its own message in addition to the system one when a core dump happens, which breaks a few tests which match on the exact output of the system.

This PR changes those tests to strip the (possible) QEMU output before checking if the output is expected.

Author: bors