tests: fix IMAGE_ROOTFS_EXTRA_SPACE typo in test_image_rootfs_extra_space
tests: fix IMAGE_ROOTFS_EXTRA_SPACE typo in test_image_rootfs_extra_space

Changelog: none
Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@rndity.com>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender