selftests/vsock: fix vsock_test path shadowing in nested VMs

The /root mount introduced for nested VM support shadows any host paths
under /root. This breaks systems where the outer VM runs as root and the
vsock_test binary path is something like:

/root/linux/tools/testing/selftests/vsock/vsock_test

Fix this by copying vsock_test into the temporary home directory that
gets mounted as /root in the guest, and using a relative path to invoke
it.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260317-vsock-vmtest-nested-fixes-v2-2-0b3f53b80a0f@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/vsock/vmtest.sh

@@ -393,6 +393,7 @@ handle_build() {
 setup_home() {
 	mkdir -p "$(dirname "${SSH_KEY_PATH}")"
 	ssh-keygen -t ed25519 -f "${SSH_KEY_PATH}" -N "" -q
+	cp "${VSOCK_TEST}" "${TEST_HOME}"/vsock_test
 }
 
 create_pidfile() {
@@ -620,7 +621,7 @@ vm_vsock_test() {
 	# log output and use pipefail to respect vsock_test errors
 	set -o pipefail
 	if [[ "${host}" != server ]]; then
-		vm_ssh "${ns}" -- "${VSOCK_TEST}" \
+		vm_ssh "${ns}" -- ./vsock_test \
 			--mode=client \
 			--control-host="${host}" \
 			--peer-cid="${cid}" \
@@ -628,7 +629,7 @@ vm_vsock_test() {
 			2>&1 | log_guest
 		rc=$?
 	else
-		vm_ssh "${ns}" -- "${VSOCK_TEST}" \
+		vm_ssh "${ns}" -- ./vsock_test \
 			--mode=server \
 			--peer-cid="${cid}" \
 			--control-port="${port}" \