bump diego-stress-tests
bump diego-stress-tests

[delivers #129111477]

Submodule src/code.cloudfoundry.org/diego-stress-tests 311e7ce..a3421c8:
  > curl apps in parallel

Signed-off-by: John Shahid <d5f6a919b5b5e6a4f7f09f1adf45e90e5c38b58e@gmail.com>

cloudfoundry-incubator/diego-perf-release,cloudfoundry-incubator/diego-perf-release