Add missing array-enlargement logic to test_regex.c.

Add missing array-enlargement logic to test_regex.c.

The stanza to report a "partial" match could overrun the initially
allocated output array, so it needs its own copy of the array-resizing
logic that's in the main loop.  I overlooked the need for this in
ca8217c10.

Per report from Alexander Lakhin.

Discussion: https://postgr.es/m/3206aace-50db-e02a-bbea-76d5cdaa2cb6@gmail.com