Cache test users to speed up testing (#13741)

* Cache test users to speed up testing

Users were recreated for many tests and hashing passwords takes a lot of
time. Testing /api4 takes around 120s less and /app 60s less.

* fix build

* simplifying a bit the approach

Co-authored-by: mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Jesús Espino <jespinog@gmail.com>

api4/user_test.go

@@ -1242,6 +1242,10 @@ func TestAutocompleteUsers(t *testing.T) {
 func TestGetProfileImage(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
+
+	// recreate basic user
+	th.BasicUser = th.CreateUser()
+	th.LoginBasic()
 	user := th.BasicUser
 
 	data, resp := th.Client.GetProfileImage(user.Id, "")