test: null terminate output buffers

Previously, the standard i/o stream and daemon logs
were not properly null terminated, which could cause
issues in `print_lines()` inside `log_test_result()`.

See #1710

test/pwtest.c

@@ -996,6 +996,12 @@ error:
 		}
 	}
 
+	for (size_t i = 0; i < SPA_N_ELEMENTS(t->logs); i++) {
+		char *e = pw_array_add(&t->logs[i], 1);
+		spa_assert_se(e);
+		*e = '\0';
+	}
+
 	close_pipes(read_fds);
 	close_pipes(write_fds);