ltp: Exclude the memcg_stress tests due to timeout problems

This test runs for 900s, we often see tests killed after 300s without
output which makes the test results unreliable and inconsistent. The
easiest solution for now is to skip this long running test, patching
it out wth sed.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>