test: add test for PARENT_SCOPE behavior

Test code courtesy of Alex Merry <alex.merry@kde.org>.
cmDefinitions: only pull variables if they aren't local

Prior to the commit which changed behavior here, PARENT_SCOPE triggered
a ->GetInternal() call which would internally pull down the parent's
value into the local scope. When converted over to an explicit ->Pull()
call, the logic in ->GetInternal() which ignored the parent if it is
already in scope was missed and instead unconditionally pulled down.

Reported-By: Ben Cooksley <bcooksley@kde.org>
Merge topic 'variable-pull-failure' into next

91112028 cmDefinitions: only pull variables if they aren't local
d770c2f5 test: add test for PARENT_SCOPE behavior