tests: fix TTY check in virTestUseTerminalColors()

Since colors would be used when writing to stdout, then check that
stdout is a TTY, instead of stdin.

This avoids the usage of terminal color codes when the output is
directed to file.

tests/testutils.c

@@ -92,7 +92,7 @@ bool virTestOOMActive(void)
 
 static int virTestUseTerminalColors(void)
 {
-    return isatty(STDIN_FILENO);
+    return isatty(STDOUT_FILENO);
 }
 
 static unsigned int