Adding an option for a default value for Mock-driven test-double

[are-scenic]

There are often cases in which a certain test-double in most of the tested is anticipated to return some default value, and only in specific dedicated tests we try out other different values.
Today, STAT-POP API that is used to implement such test-double requires doesn't allow default values, but always requires a Mock to be created for each call. Adding the ability of defining default values might improve the developer experience greatly.

What do you think?