selftests/vsock: make wait_for_listener() work even if pipefail is on

selftests/vsock: make wait_for_listener() work even if pipefail is on

Rewrite wait_for_listener()'s pattern matching to avoid tripping the
if-condition when pipefail is on.

awk doesn't gracefully handle SIGPIPE with a non-zero exit code, so grep
exiting upon finding a match causes false-positives when the pipefail
option is used (grep exits, SIGPIPE emits, and awk complains with a
non-zero exit code). Instead, move all of the pattern matching into awk
so that SIGPIPE cannot happen and the correct exit code is returned.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251108-vsock-selftests-fixes-and-improvements-v4-2-d5e8d6c87289@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>