example: Provide ProgramTest example with spl-token CPI

[joncinque]

Program developers will want to write end-to-end tests using other programs, which is not totally possible with unit tests. solana-program-test is extremely useful, so we should have a small example of how to use it. A few things to consider:

• an example of adding another program's processor function (e.g. spl_token)
• an example using a builtin (e.g. stake)

Also, there's an error that I ran into while doing this:
error: the #[global_allocator] in solana_measure conflicts with global allocator in: spl_stake_pool

so an extra comment or an FAQ point about that error and how to resolve it would be useful as well, e.g. adding
#![cfg(all(target_arch = "bpf", not(feature = "no-entrypoint")))] in entrypoint.rs to avoid defining global_allocator twice.

tag @jackcmay

[jackcmay]

@joncinque

• What do you think we should do something simple like two test programs, once calls the other to keep the example super clean (rather then call token)?
• Does the current cross-program-invocation example cover the builtin example (it calls system program

[mvines]

I definitely think this example should invoke SPL Token. The first thing people familiar with ERC-20 ask seems to be, "so how do I customize the SPL Token implementation for my application". To which we reply, "please don't, there's a better way".

I conveniently wrote this example earlier today, mvines/solana-bpf-program-template#3, that perhaps can be recycled in whole or part for this issue

[jackcmay]

@mvines What was the intent of writing that example in its own repo and not as an example under SPL?

[mvines]

Nope. Just wanted to park it somewhere for now, and seemed easiest to add it under my account for now.

[jackcmay]

Cool, I'll add it to the examples

[joncinque]

@jackcmay my bad, I put this in late yesterday and hadn't read through all of the examples you've already provided. the cross-program-invocation program is perfect as it is for using builtins. I was thinking the same thing as @mvines regarding an spl-token CPI example, since most people want to work with tokens.

[jackcmay]

@joncinque the global allocator conflict should go away now that SPL is on v1.4.8. I remove the extra checks in this PR: #852