lib: tests: Move tests to a separate directory

Move all of the SBIUnit-related code into the lib/sbi/tests directory.
Update 'Makefile' to index objects from the tests subdirectory.

I don't think creating the full separate list of Makefile variables
(libsbitests-objs-path-y, libsbitests-object-mks, etc. as it is done for
libsbiutils) is necessary for the tests because:

1) `lib/sbi/tests/objects.mk` is already indexed into
'libsbi-objects-mks' since the find expression for the libsbi-object-mks
variable looks for objects.mk files in the nested directories as well).

2) Tests are tightly coupled with the `lib/sbi/` sources, therefore it
may be reasonable to store the list of lib/sbi and lib/sbi/tests object
files together in the libsbi-objs-path-y variable.

Additionally, update relative paths in the tests where necessary.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>