Simplify bin testing

[This piece of code](https://github.com/sindresorhus/flow-bin/blob/71384dcc2c7c70cf9c49bd4c3cc61a1f4327ea5d/lib/install.js#L5-L13) can be used in most scenarios to test the binary. Would be nice if I could instead just call a method on `bin-wrapper` to do that, so to reduce useless boilerplate.

Start by reading the linked lib/install.js lines 5-13 to understand the repeated binary-testing pattern and then inspect bin-wrapper's public API and existing tests. Done means the common test setup can be performed through bin-wrapper without the described boilerplate, with coverage for the supported usage.

Written by the indexing model from the issue text.