Avoid duplication in metadata property check tests (#31)
Avoid duplication in metadata property check tests (#31)

The code that tests that a property gets read in the resulting image
metadata for both `.getFromFilePath()` and `.getImageMetadata()` is
duplicated a lot of times, and can be error prone, if for example, you
add tests for `.getFromFilePath()` but forget to ensure the property is
also passed in `.getImageMetadata()`.

Signed-off-by: Juan Cruz Viotti <4148f5869aa1bcd7efd85b513a5874967e585032@openmailbox.org>
resin-io-modules/etcher-image-stream