I cannot login to a user I just created in a PDB
You are connecting to root container by using sqlplus testtest/password where the user doesn't exist. 

Instead, you can use EZConnect or you can create a TNS name to connect to the PDB.

Example:

EZCONNECT

[oracle@host ~]$ sqlplus testtest@hostname:port/bpdb1


TNS Name

First, find out the service name of the PDB, one of the methods has been shown below.

 [oracle@host ~]$ lsnrctl status listener_name

Service "orclpdb" has 1 instance(s).
  Instance "oracdbuat", status READY, has 1 handler(s) for this service...


In my case, it's orclpdb.
Go to $ORACLE_HOME/network/admin.
Edit/Create tnsnames.ora file to add a similar entry as shown below.

ORCLPDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = hostname)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = orclpdb)
    )
  )


Now, I can connect to the PDB using the TNS name.

[oracle@host ~]$ sqlplus testtest@orclpdb