test: add test for PARENT_SCOPE behavior

Test code courtesy of Alex Merry <alex.merry@kde.org>.

Tests/RunCMake/set/ParentPulling-stderr.txt

+^before PARENT_SCOPE blah=value2
+after PARENT_SCOPE blah=value2
+in parent scope, blah=value2$

Tests/RunCMake/set/ParentPulling.cmake

+cmake_minimum_required(VERSION 3.0)
+project(Minimal NONE)
+
+function(test_set)
+    set(blah "value2")
+    message("before PARENT_SCOPE blah=${blah}")
+    set(blah ${blah} PARENT_SCOPE)
+    message("after PARENT_SCOPE blah=${blah}")
+endfunction()
+
+set(blah value1)
+test_set()
+message("in parent scope, blah=${blah}")

Tests/RunCMake/set/RunCMakeTest.cmake

 include(RunCMake)
 
 run_cmake(PARENT_SCOPE)
+run_cmake(ParentPulling)