string: add another test for split_nth()

vlib/builtin/string_test.v

@@ -145,6 +145,11 @@ fn test_split_nth_values() {
 	assert a3[1] == 'eprintln(phase'
 	assert a3[2] == '1)'
 
+	a4 := line.split_nth('=', 4)
+	assert a4.len == 3
+	assert a4[0] == 'CMD'
+	assert a4[1] == 'eprintln(phase'
+	assert a4[2] == '1)'
 }
 
 fn test_split() {