Better feedback for ava test

[conartist6]

For a while now I've been writing ava test somefile.js. As I also use ava debug I assumed that this tool followed the subcommand pattern. Instead what I now realize is that there is no test subcommand -- I've just been filtering test based on two patterns, one that exists and one that does not. I propose that if the users types ava test and no tests match the pattern, a warning is displayed which suggests just running ava.