New unit tests to demonstrate bug

Output from go test before:
```
--- FAIL: TestNextTickAfter (0.01s)
    --- FAIL: TestNextTickAfter/next_run_after (0.01s)
        --- FAIL: TestNextTickAfter/next_run_after/next_run_after_excl_#63:_0_0_*_*_1#1 (0.00s)
            next_test.go:66: [0 0 * * 1#1][2009-11-23 00:00:00] expected 2009-12-07 00:00:00, got 2010-01-04 00:00:00
```

Now it all passes

gronx_test.go

@@ -207,6 +207,8 @@ func testcases() []Case {
 		{"* * * * 5#2", "2011-07-01 00:00:00", false, "2011-07-08 00:00:00"},
 		{"* * * * 5#1", "2011-07-01 00:00:00", true, "2011-07-01 00:01:00"},
 		{"* * * * 3#4", "2011-07-01 00:00:00", false, "2011-07-27 00:00:00"},
+		{"0 0 * * 1#1", "2009-10-23 00:00:00", false, "2009-11-02 00:00:00"},
+		{"0 0 * * 1#1", "2009-11-23 00:00:00", false, "2009-12-07 00:00:00"},
 		{"5/0 * * * *", "2021-04-19 12:54:00", false, "2018-08-13 00:25:00"},
 		{"5/20 * * * *", "2018-08-13 00:24:00", false, "2018-08-13 00:25:00"},
 		{"5/20 * * * *", "2018-08-13 00:45:00", true, "2018-08-13 01:05:00"},

validator.go

@@ -142,5 +142,5 @@ func isValidWeekDay(val string, last int, ref time.Time) (bool, error) {
 		return false, nil
 	}
 
-	return ref.Day()/7 == nth-1, nil
+	return (ref.Day()-1)/7 == nth-1, nil
 }