docs: writing_tests: Make docs correspond the latest changes

We should store test object files list in the `libsbi-objs-y` Makefile
variable, not in `libsbitests-objs-y`. Update the documentation
correspondingly.

Since we don't use the `console_dev` static variable directly in the
`sbi_console_test` unit test anymore, remove the paragraph which says
that we do.

Fixes: 86224ec3

 ("docs/writing_tests: Update tests paths")
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>