[patch, testsuite] Committed test for PR 20520

[Thomas Koenig]

Hi,

I just committed the test case below so the bug can be closed.

The error message is less than ideal, but at least it is something.
Maybe I'll file a separate PR about translating error messages about
artificial variables to user's variables.

Regards

Thomas

2018-10-31 Thomas Koenig <***@gcc.gnu.org>

PR fortran/20520
* gfortran.dg/allocatable_uninitialized_1.f90: New test.