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.inf

@@ -0,0 +1,25 @@
+## @file
+#  Mock implementation of the UEFI Runtime Services Table Library.
+#
+#  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = MockUefiRuntimeServicesTableLib
+  FILE_GUID                      = 4EA215EE-85C1-4A0A-847F-D2A8DE20805F
+  MODULE_TYPE                    = UEFI_DRIVER
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = UefiRuntimeServicesTableLib|HOST_APPLICATION
+
+#
+#  VALID_ARCHITECTURES           = IA32 X64 EBC
+#
+
+[Sources]
+  MockUefiRuntimeServicesTableLib.c
+
+[Packages]
+  MdePkg/MdePkg.dec