What is the test for disqualification as a witness?

" 'As a general rule, "every person, irrespective of age, is qualified to be a witness and no person is disqualified to testify to any matter." (Evid. Code, 700; see Pen. Code, 1321.) A person may be disqualified as a witness for one of two reasons: (1) the witness is incapable of expressing himself or herself so as to be understood, or (2) the witness is incapable of understanding the duty to tell the truth. (Evid. Code, 701, subd. (a).) The party challenging the witness bears the burden of proving disqualification, and a trial court's determination will be upheld in the absence of a clear abuse of discretion. [Citation.]' " (People v. Dennis (1998) 17 Cal.4th 468, 525, quoting People v. Mincey (1992) 2 Cal.4th 408, 444.)