Investigate Linux-only line-break failures

[gsnedders]

Post #10913, we have a lot of the line break tests failing only on Linux. @mbrubeck speculated that this was down to font availability.

We should:

• Check this is indeed the cause,
• Make sure we have fonts on CI that cover it.

I'm happy to work on this, but I'm away for a few weeks still. :)

[emilio]

I opened word-break-normal-bo-000.html, and here are the results. Just out of curiosity compared with other browsers.

How does the test look in OS X?

Servo

Ref:

Gecko

Ref:

Chromium

[shssoichiro]

It doesn't look any better for me on OS X 10.11

[dralley]

Test passes for me:

However it still looks different from Gecko