Fix integer constant overflows in tests

* Use integer limit value [1] instead of hard-coded magic constant for
  NonexistentID.

* Explicitly use int64 in order to avoid the following errors on 32 bit
  architectures:

  # code.gitea.io/gitea/integrations
  ./api_admin_test.go:50:34: constant 9223372036854775807 overflows int
  ./api_token_test.go:47:34: constant 9223372036854775807 overflows int
  [...]
  # code.gitea.io/gitea/models
  ./action_test.go:179:15: constant 9223372036854775807 overflows int

[1] https://golang.org/pkg/math/#pkg-constants

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>

models/unit_tests.go

@@ -7,6 +7,7 @@ package models
 import (
 	"fmt"
 	"io/ioutil"
+	"math"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -23,7 +24,7 @@ import (
 )
 
 // NonexistentID an ID that will never exist
-const NonexistentID = 9223372036854775807
+const NonexistentID = int64(math.MaxInt64)
 
 // giteaRoot a path to the gitea root
 var giteaRoot string