enable previously leftout test_usetex

[jenshnielsen]

Follow up to #6917 with the tests only on master

[jenshnielsen]

Have not had the time to investigate why the png test fails. It passes with that image for me locally.

[jenshnielsen]

Reproduced the failure locally on a Ubuntu 12.04 vm. The text looks correct but is slightly different. I don't think this is a font issue so it is probably due to different tex versions.
Attached diff between travis and current reference from my OSX machine

[jenshnielsen]

This still need a skip if not latex decorator

[jenshnielsen]

I think this is ready to land now

[Kojoley]

@jenshnielsen please try to rearrange extension order. If it will pass with ['png', 'pdf'] then this is #6899 bug.

[jenshnielsen]

@Kojoley Good point, I will give it a try

[Kojoley]

Why not to use skip_if_command_unavailable decorator?

[jenshnielsen]

Please read the commit message

[Kojoley]

Please read the commit message

The actual problem commit is gone there and only comment left. :(

At least you can simplify this with matplotlib.checkdep_tex() call.

[jenshnielsen]

I have updated this to use checkdep_usetex which is what we want

[jenshnielsen]

I think this is ready to merge, the change in coverage seems to be a fluke. The tolerance does not seem to depend on the order it is passing and failing on different backends because of tex version differences. I can reproduce the image that matches travis on a ubuntu 12.04 vm (same os and texlive version as travis) but get different results with a modern texlive install.