Docs: Move test related docs to tf-m-tests repo

It can be more clear to put test partition and suites addition
documents under tf-m-tests repo to distinguish with secure partition
addition content.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I8d746d734ac527e60a5a98508cb6b7700fe7b95b
3 files changed