mod_power_domain/unit_test: fix compiler error

On gcc 13.2.0 the following error is flagged:
error: comparison of integer expressions of different signedness:
‘int’ and ‘enum pd_idx’ [-Werror=sign-compare]

Positive enum should be compared against unsigned variables.

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