tests: Increase test Dockers' memory limit to 18GB

The following error is being returned by the Jenkins,
https://jenkins.gem5.org, on a regular basis:

```
error waiting for container: unexpected EOF
```

An example can be found here:
https://jenkins.gem5.org/job/nightly/172/console

It is not known exactly what the cause of this error is, but increasing
the memory available to each container may help the issue. This patch
may be reverted if it does not fix the problem.

Change-Id: Ide341774439820def91b0373b03801396705cd62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58169
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed