This implements the following cases:
* Basic runs for a specific version/platform/arch with default options
(testing for existence of expected output files/folders)
* Runs for each target platform with `out` set
* Runs for each target platform with `asar` set
* Runs for each target platform with `prune` set
* Runs for each target platform with `ignore` set
* This includes testing `ignore` including a path separator
* This also includes testing that `ignore` entries are applied
with the app folder trimmed
* Runs for each target platform with `overwrite` set / not set
* Runs with `all` set, with `arch` or `platform` each set to `all`,
and with `platform` set to multiple platforms (comma-delimited or
array)
* Tests specifically for the darwin target platform, to test
`icon` and `sign`
The test logic is organized such that the version of Electron being
tested can be configured once centrally (in config.json), and the tests
will not run until after downloading any necessary electron versions
(to avoid timing out due to long downloads).
Resolves#77.
