test: fix unit tests failing on repeated runs (#39915)

`go test -count=2` failed in 12 packages because tests leaked state into
later runs.

- Reset SQLite sequences after each fixture load, like PostgreSQL
already does
- Prepare the database in tests that relied on earlier tests doing it
- Restore mutated globals with `test.MockVariableValue`

models/user/user_test.go

@@ -128,6 +128,8 @@ func TestUserEmails(t *testing.T) {
 }
 
 func TestCanCreateOrganization(t *testing.T) {
+	defer test.MockVariableValue(&setting.Admin.DisableRegularOrgCreation)()
+
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
 	admin := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})