What is the test for a motion to withdraw a plea of guilty under section 1237.5 of the California Criminal Code?

'"It is settled that the right of appeal is statutory and that a judgment or order is not appealable unless expressly made so by statute.'" (People v. Mazurette (2001) 24 Cal.4th 789, 792.) Section 1237.5 provides that "[n]o appeal shall be taken by the defendant from a judgment of conviction upon a plea of guilty or nolo contendere . . . except where . . . [] . . . [] (b) [t]he trial court has executed and filed a certificate of probable cause . . . ." Section 1237.5, subdivision (b), "does not limit the scope of review of the denial of a motion to withdraw a plea of guilty when that error is properly before the court on appeal. It merely sets forth a procedure for precluding frivolous appeals by requiring the defendant to set forth grounds for appeal and, if he does so, by requiring the trial court to rule on the issue of probable cause." (People v. Ribero (1971) 4 Cal.3d 55, 62.) Before its enactment, "the mere filing of a notice of appeal required preparation of a record and, in many cases, appointment of counsel; only after expenditure of those resources would an appellate court determine whether the appeal raised nonfrivolous issues that fell within the narrow bounds of cognizability. Section 1237.5[, subdivision (b),] was intended to remedy the unnecessary expenditure of judicial resources by preventing the prosecution of frivolous appeals challenging convictions on a plea of guilty." (People v. Hoffard (1995) 10 Cal.4th 1170, 1179.)