Add a filesystem interface, mock, and tests for the mock.

This allows testing sysfs based drivers by swapping out the real
filesystem for a mock one.

Signed-off-by: Michael Hope <mlhx@google.com>
arietta: switch all sysfs drivers to use the fs interface.

Allows unit testing by mocking out the filesystem.

Signed-off-by: Michael Hope <mlhx@google.com>
arietta: add adaptor, digital pin, and PWM tests.

Signed-off-by: Michael Hope <mlhx@google.com>