selftests: ifs: verify test image loading functionality

Scan test image files have to be loaded before starting IFS test.

Verify that In Field scan driver is able to load valid test image files.

Also check if loading an invalid test image file fails.

Reviewed-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Co-developed-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Pengfei Xu <pengfei.xu@intel.com>
Acked-by: Jithu Joseph <jithu.joseph@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>