Allow names other than t for ava execution context

[DavidTanner]

Updates to:

• ava parser

In my company we often use the following, and I would much rather update this lib than fix all the tests by hand.

import test from 'ava';

test('some test', (test) => {
  ...
});

[skovhus]

Thanks for your contribution! 👏

Let me know if you find more cases we should handle while migrating the ava codebase.