[gardener] Convert crash test to async_test

This test is flaky because we get a variable number of CONSOLE WARNINGs
in the output. The exact output is not important for a crash test,
hence this CL converts it to a testharness test which ignores
the output.

Change-Id: Id7f09db68927533e41241c45bcd22a7a5fcb754e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6971552
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518628}