sim: Add a ProxyPtr test.

Change-Id: If71cc374030a5ef0dab62d351bc83960ff509af7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29401
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/SConscript

@@ -82,6 +82,7 @@ Source('power_domain.cc')
 
 GTest('byteswap.test', 'byteswap.test.cc', '../base/types.cc')
 GTest('guest_abi.test', 'guest_abi.test.cc')
+GTest('proxy_ptr.test', 'proxy_ptr.test.cc')
 
 if env['TARGET_ISA'] != 'null':
     SimObject('InstTracer.py')