ignore non test files, fixes #209

[GabiGrin]

I took a long shot with this PR but hopefully it helps :)

This fix has 2 parts:

1. When a file that didn't include AVA ran inside the babel.js child process the process will never exit as no piece of code will ever emit the needed events.
   The only way I found around this, because of the complex process nature of the code, is to send a message once AVA runs on a file, and another message when babel finishes. The result is that inside fork.js we can now know if a file that did not include AVA was read and exit if so.
   PS: this only happens in master, not in v0.4.2, probably because the fix in here

2. If a file didn't include AVA, test results are not emitted and thus an the error in the mentioned issue is is thrown. I filtered the all undefined results to fix this.

Also, added a test that will fail on master / latest published version.

[jamestalmage]

IMO, the proper thing to do in a file without tests is to fail. Helper files should be prefixed with a _ character.

The only way I found around this, because of the complex process nature of the code, is to send a message once AVA runs on a file, and another message when babel finishes

See #208 for my solution to the problem. It's equally convoluted, but does not require sending messages between processes.

[GabiGrin]

Much more elegant!

I actually got into this issue while running AVA on directly on a folder (via gulp-ava) with other files, but that was just by mistake.

I'll close thi. Thanks!