Fortran, testsuite: Add testcases for nested structures.

As as result of printing only the outer elements of nested structures,
some testcases have to be added to check for corner cases with VLA's.

2016-05-25  Bernhard Heckel  <bernhard.heckel@intel.com>

gdb/testsuite/Changelog:
	* gdb.fortran/vla-type.exp: Access elements in nested structs.