Add more error info to TestDockerRunEchoStdoutWithMemoryLimit

This test fails on my CI server often so we need more info when it does
happen with this test.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Merge pull request #7833 from crosbymichael/mem-test

Add more error info to TestDockerRunEchoStdoutWithMemoryLimit