MdePkg: Add gmock examples

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4389

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Chris Johnson <chris.n.johnson@intel.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Oliver Smith-Denny <osde@linux.microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

MdePkg/MdePkg.dec

@@ -27,6 +27,7 @@
 [Includes]
   Include
   Test/UnitTest/Include
+  Test/Mock/Include
 
 [Includes.IA32]
   Include/Ia32