Modify Linux travis config to run content and cpu ref tests.

[glennw]

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[glennw]

r? @larsbergstrom @metajack

[larsbergstrom]

r+

[mrobinson]

Hi @glennw. I'm trying to debug a new failure that only happens on the dummy X server on travis (#3245). The problem I'm having is that the current setup seems to require root and takes over the monitor when run manually. Any reason you didn't go with Xvfb here, which can be run nested and invisible inside a pre-existing X session? Do you know a technique to use the current setup in a way that's friendly to the current X session?