Fix Setup.php file-scope test

Fix Setup.php file-scope test

Way back in r8196, a test was added to assert that Setup.php is being
included at file scope.[1] That test was broken when I633a6ff23 moved the
inclusion of DefaultSettings.php into Setup.php itself.

This fixes the test by having it directly set a variable and see if that
variable is a global. Yes, there's several ways to bypass it, but we're
just trying to avoid accidents here rather than trying to be foolproof.

[1]: Or really, in the same scope as DefaultSettings.php. But it
intended to be checking for file scope.

Change-Id: I9b5f15bf01c2903e8d44d85c54465d90c047cb51