MdeModulePkg/Variable/RuntimeDxeUnitTest: Fix 32-bit GCC builds

MdeModulePkg/Variable/RuntimeDxeUnitTest: Fix 32-bit GCC builds

When using will_return() on a pointer value, it must be
cast to UINTN to be compatible with 32-bit GCC builds.
This uses the same approach in samples provided in the
UnitTestFramworkPkg when passing pointer values to
UT_ASSERT_EQUAL().

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>