libfiemap: Add a test case for larger block size filesystems.

This adds a test case for filesystems with larger than 4KiB alignment.
It creates a temporary FAT filesystem and attempts to create a backing
image. The actual size of the block device should be the requested size.

Test: fiemap_image_test
Bug: N/A
Change-Id: I8ef574b22dbf6d47f4855b7de681c927cf81f2b7