[SalesForce] Setting Password via APEX

I had this same issue. What I did is add one more code to render the user id again.

User u = new User();
u.lastname= 'blabla';
u.ContactId = currentContact.Id;
u.email='blabla@blabla.com';
...
...
...
system.setPassword(u.Id, 'password123');

User uu =[select id from user where email='blabla@blabla.com'];

return uu.id;

You need to actually find a Profile that has that permission enabled. You cannot modify Profile permissions in your tests.

One alternative solution if you do not have any Profile set up with that permission (or simply wish to use another) is to create a Permission Set. Then you could create a PermissionSetAssignment for the User in question.

PermissionSet p = [SELECT Id FROM PermissionSet WHERE DeveloperName = 'API_Only'];
insert new PermissionSetAssignment(AssigneeId=u.Id, PermissionSetId = p.id);