MdeModulePkg: Add DxeResetSystemLib unit test

https://bugzilla.tianocore.org/show_bug.cgi?id=2505

* Add unit test of DxeResetSystemLib library
  instance that uses cmocka interfaces to mock the
  UEFI Runtime Services Table and its ResetSystem()
  service.  When a unit test uses the cmocka
  interfaces, the unit test does not support being
  run from target environments.

  cmocka APIs: https://api.cmocka.org/index.html

  This example puts the unit test in a UnitTest
  directory below the library INF file and this location
  means the unit test is only designed to work this
  this one library instance.

* Add Test/MdeModulePkgHostTest.dsc to build host
  based unit tests

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Bret Barkelew <Bret.Barkelew@microsoft.com>

MdeModulePkg/Library/DxeResetSystemLib/UnitTest/MockUefiRuntimeServicesTableLib.c

@@ -0,0 +1,13 @@
+/** @file
+  Mock implementation of the UEFI Runtime Services Table Library.
+
+  Copyright (C) Microsoft Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Uefi.h>
+
+extern EFI_RUNTIME_SERVICES  MockRuntime;
+
+EFI_RUNTIME_SERVICES  *gRT = &MockRuntime;