How to generate key prefix for "unstable" types

Question

I use a mocking framework that exposes a method for generating mock record ids. The utility method uses getKeyPrefix() to mock an Id. This works for most object types but doesn't work for __Share records.

I'm not sure how to mock these Ids because even though they are "unstable" salesforce somehow can validate the id when setting it and throws an error

Folder__Share testFolderShare1 = new Folder__Share(
    Id = TestUtils.generateId(Folder__Share.SObjectType)
);

System.TypeException: Invalid id value for this SObject type: 02f000000000007AAA

How can I generate the key prefix for unstable types?

Util for generating Id:

private static Integer seed = 1;
public static Id generateId(SObjectType objectType) {
    String result = String.valueOf(seed++);
    String prefix = objectType.getDescribe().getKeyPrefix();
    if (prefix == null && objectType.getDescribe().getAssociateEntityType() == 'Share') {
        prefix = '02c';
    }
    return prefix + '0'.repeat(12 - result.length()) + result;
}