Add explicit CCK download to make PHP acceptance tests run

[ciaranmcnulty]

Fixes #134

End to end tests were skipped because the path to CCK sample data was broken during extraction from the monorepo.

• adds a separation between unit tests and the (one) end to end test
• makes the end to end test fail loudly if the CCK samples aren't present
• explicitly downloads the CCK samples from GitHub during the docker build

[luke-hill]

@ciaranmcnulty - We're releasing a major version of the CCK (v13), soon.

95% of it is ruby breaking changes, but there is one global change (2x bugfixes to attachment).

If you want to follow it: Progress is here: cucumber/compatibility-kit#52

[luke-hill]

I've had a thought about this recently, and wouldn't it be better to just have php conformance inside the CCK proper? You could even slowly introduce it for say 1 sample at a time if there are things which aren't quite right.

[mpkorstanje]

@luke-hill dependency wise that doesn't work. Using the cck to test messages creates a circular dependency. And testing specific implementation in the CCK is also something we don't want to do to keep the CCK simple.

Do keep in mind that this pull request is there to make sure tests are ran at all. It is not the perfect solution. The perfect solution would be to pare down the tests in messages just to the point where they test if serialization works. Because all messages are generated, a few well chosen tests can validate that all messages work.

So if possible, consider merging this as and then we can improve from there.