selftest/containerimage.py: Add container IMAGE_FSTYPES test

selftest/containerimage.py: Add container IMAGE_FSTYPES test

This test checks to make sure only the files expected exist in a
container image. Currently only ROOTFS_BOOTSTRAP_INSTALL, gets added to
all images without the user specifying it.

But this test should help if a developer in the future ever silently
adds more than just ROOTFS_BOOTSTRAP_INSTALL, and that the developer can
make sure it also gets removed from a container image.

[YOCTO #9502]

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>