Correctly handle test durations of more than 2147s in pg_test_timing.

Previously the computation of the total test duration, measured in
microseconds, accidentally overflowed due to accidentally using signed
32bit arithmetic.  As the only consequence is that pg_test_timing
invocations with such, overly large, durations never finished the
practical consequences of this bug are minor.

Pointed out by Coverity.

Backpatch to 9.2 where pg_test_timing was added.

contrib/pg_test_timing/pg_test_timing.c

@@ -115,7 +115,7 @@ test_timing(int32 duration)
 				end_time,
 				temp;
 
-	total_time = duration > 0 ? duration * 1000000 : 0;
+	total_time = duration > 0 ? duration * INT64CONST(1000000) : 0;
 
 	INSTR_TIME_SET_CURRENT(start_time);
 	cur = INSTR_TIME_GET_MICROSEC(start_time);