CrmScopeTest - Use explicit value for default/global example

CrmScopeTest - Use explicit value for default/global example

Before: We rely on default/global value of `x=UNDEFINED` or `x=NULL`.
But this is a bit tempermental.

After: Clear default.

Note: There's a prior commit which adds another test (CRM_Core_SmartyTest)
where the global value is specifically undefined or specifically null.
So that still has coverage.