Created faking and mocking functions

[v-maliutin]

I developed some functions to make writing tests easier. There are examples in the source code. Also, I wrote an article on habr. Here it is https://habr.com/ru/companies/gnivc/articles/877314/. I know it is in Russian, but hope modern translation will help.
Anyway such functionality is in any modern language and wide known. I just bringing it in PG world.
One more thing. I am going to introduce my work and pgTap at pgConf in Moscow on March 31 2025.

[v-maliutin]

@theory Hi! Could you please let me know what you think about my PR. I'd appreciate it.

[theory]

Sorry, @v-maliutin, hyper busy rn, but will get to it.

[jnasbyupgrade]

Not a full review, but a couple comments:

AFAIK, there's no reason to modify the unpackaged script.

Instead of building a function body via ||, I think it would be a lot clearer to use format().

[v-maliutin]

Not a full review, but a couple comments:

AFAIK, there's no reason to modify the unpackaged script.

Instead of building a function body via ||, I think it would be a lot clearer to use format().

@jnasbyupgrade thank you for your review. Rolled back changes in Unpackaged
Regarding format vs ||. May I stay as is? Just I always lost in '%1L, %2I, %3s'. Especially when there are a lot of %. Anyway I can fix it if you insist. This is not a big problem for me.