platform/model: Don't pull in the "testing" package in normal builds (#6735)

Rename all files importing the "testing" package in "platform/model"
so that they only get compiled in test builds. This avoids the side
effects of the testing package (notably, defining some command line
flags with the "flags" package) for codebases importing the model
package.

model/modeltestlib_test.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2017 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"runtime/debug"
+	"testing"
+)
+
+func CheckInt(t *testing.T, got int, expected int) {
+	if got != expected {
+		debug.PrintStack()
+		t.Fatalf("Got: %v, Expected: %v", got, expected)
+	}
+}
+
+func CheckInt64(t *testing.T, got int64, expected int64) {
+	if got != expected {
+		debug.PrintStack()
+		t.Fatalf("Got: %v, Expected: %v", got, expected)
+	}
+}
+
+func CheckString(t *testing.T, got string, expected string) {
+	if got != expected {
+		debug.PrintStack()
+		t.Fatalf("Got: %v, Expected: %v", got, expected)
+	}
+}
+
+func CheckTrue(t *testing.T, test bool) {
+	if !test {
+		debug.PrintStack()
+		t.Fatal("Expected true")
+	}
+}
+
+func CheckFalse(t *testing.T, test bool) {
+	if test {
+		debug.PrintStack()
+		t.Fatal("Expected true")
+	}
+}
+
+func CheckBool(t *testing.T, got bool, expected bool) {
+	if got != expected {
+		debug.PrintStack()
+		t.Fatalf("Got: %v, Expected: %v", got, expected)
+	}
+}