SSSDConfigTest: Use unique temporary directory
    
    Test SSSDConfigTest is executed twice with make check
    if python2 and python3 are available.
    Tests are executed in parallel with new automake
    and therefore it caused sometimes failures e.g.
    
    ERROR: testModifyExistingConfig (__main__.SSSDConfigTestValid)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/tmp/sssd/src/config/SSSDConfigTest.py", line 215, in testModifyExistingConfig
        mode = os.stat(of)[ST_MODE]
    FileNotFoundError: [Errno 2] No such file or directory: '/tmp/testModifyExistingConfig.conf'
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>