Fix test output so that filenames have the correct path.

Tested:
  Intentionally introduce an error and then run:
  blaze test --test_output=streamed //third_party/gvisor/test/packetimpact/tests:tcp_outside_the_window_linux_test
PiperOrigin-RevId: 308114194

test/packetimpact/tests/test_runner.sh

@@ -262,7 +262,10 @@ sleep 3
 # Start a packetimpact test on the test bench.  The packetimpact test sends and
 # receives packets and also sends POSIX socket commands to the posix_server to
 # be executed on the DUT.
-docker exec -t "${TESTBENCH}" \
+docker exec \
+  -e XML_OUTPUT_FILE="/test.xml" \
+  -e TEST_TARGET \
+  -t "${TESTBENCH}" \
   /bin/bash -c "${DOCKER_TESTBENCH_BINARY} \
   ${EXTRA_TEST_ARGS[@]-} \
   --posix_server_ip=${CTRL_NET_PREFIX}${DUT_NET_SUFFIX} \