The testimony of each witness in a proceeding concerning the execution or provisions of a will, the testamentary capacity of the decedent, and other issues of fact, may be reduced to writing, signed by the witness, and filed, whether or not the will is contested. The testimony so preserved, or an official reporter’s transcript of the testimony, is admissible in evidence in any subsequent proceeding concerning the will if the witness has become unavailable as a witness within the meaning of Section 240 of the Evidence Code.