test: add test and refactor for fwk_module_bind

Comments have been added to clarify the tests. The tests have been
moved at the end of the file to follow the declaration order of the
functions in fwk_module.h.

Sub-tests have been added to cover missing paths.

Change-Id: I0c22a76d87e093db151894714e7c80fbb14a8e19
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>