MdeModulePkg/NullMemoryTest: Handle more reliable memory

MdeModulePkg/NullMemoryTest: Handle more reliable memory

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=650

Today's implementation converts the untested more reliable memory
from reserved GCD type to system memory GCD type.
Though it doesn't impact the return result of gBS->GetMemoryMap().
But it impacts the return result of gDS->GetMemorySpaceDescriptor().
The patch fixes the bug to convert the untested more reliable memory
from reserved GCD type to more reliable memory GCD type.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>