Fix format truncation issue from ECPG test

This fixes one warning generated by GCC and present in the test case
array part of ECPG.  This likely got missed in past fixes like 3a4b891
because the compilation of those tests is not done by default.

Reported-by: Sergei Kornilov
Discussion: https://postgr.es/m/14951331562847675@sas2-a1efad875d04.qloud-c.yandex.net

src/interfaces/ecpg/test/expected/sql-array.c

@@ -156,7 +156,7 @@ main (void)
 	ECPGdebug(1, stderr);
 
 	for (j = 0; j < 10; j++) {
-		char str[20];
+		char str[28];
 		numeric *value;
 		interval *inter;
 

src/interfaces/ecpg/test/sql/array.pgc

@@ -34,7 +34,7 @@ EXEC SQL END DECLARE SECTION;
 	ECPGdebug(1, stderr);
 
 	for (j = 0; j < 10; j++) {
-		char str[20];
+		char str[28];
 		numeric *value;
 		interval *inter;