amu_smcf_drv/unit_test: Fix compiler error

On gcc 13.2.0 the following error is flagged:
storing the address of local variable ‘data_api’ in
‘amu_smcf_drv_ctx.data_api’ [-Werror=dangling-pointer=].

Storing a local pointer in a global variable should be avoided.

Signed-off-by: Tarek El-Sherbiny <tarek.el-sherbiny@arm.com>