Blocks #78.

We need a way to do static analysis of test files, specifically to discover if any use exclusive tests (i.e. test.only).

There are plenty of complications:

1. Naming the AVA import something other than test:

   import ava from 'ava';

2. Storing a reference to a test.only for reuse:

   const shortcut = test.serial.only;
   
   shortcut(t => t.is(...)); // will be an exclusive, serial test

   Similar can also be achieved with import statements:

   import test, {skip, todo, only} from 'ava';

   Or require statements:

   var skip = require('ava').skip;

3. Dynamically generating tests (see [Idea]: test macros #695 possible solution).

   function testAddition(a, b, expected) {
    test(`${a} + ${b} === ${expected}`, t => t.is(a + b, expected));
   }
   
   testAddition(2, 2, 4);
   testAddition(3, 4, 7);

4. Non-constant variables (variables that are reassigned after declaration, not referring to const here):

   var test;
   test = require('ava');

   ---

It seems possible to support 1 and 2 without a lot of pain. 3 quickly becomes impossible. I think #695 provides a solution that would allow us to cover most use cases for dynamically generating tests and still give us the ability to do the static analysis we need to. 4 just seems silly and probably not very likely in practice.