testsuite: gnat.dg: fix dg-do directive syntax

Fix incorrect use of '[' rather than '{' in 'dg-...' directives.

gcc/testsuite/ChangeLog:

	* gnat.dg/pack13.adb: Fix 'dg-...' directive syntax.
	* gnat.dg/size_attribute.adb: Ditto.
	* gnat.dg/subp_elim_errors.adb: Ditto.