Integration tests (experimental runtime tests framework)

[lemunozm]

Description

Fixes #1458 #1375 #1499

This PR adds a "new way" of doing integration tests, but only when the purpose is to test the interaction between different pallets in runtimes. No node, no fudge, no 3parties, no xcm, just pallet interactions in real runtimes. Slack thread with the initial discussion.

In order to improve compilation times and testing feedback loop, I've created a new crate under runtime/tests where tests can be compiled much faster. cc @mustermeiszer

Motivation

Create a way to test end-to-end paths in a runtime context independently of the runtime used.

Example. The following parameterized test:

fn test_foo<T: Config>() {
    // Test over a T runtime configuration common to all our runtimes,
    // with some common instantiations to improve the usage:
    // Balance as u128, CurrencyId, AccountId32, ...
    
    // ...
    
    // If the test requires different choices for each runtime, you can dispatch as follows:
    match T::KIND {
        RuntimeKind::Development => ...,
        RuntimeKind::Altair => ...,
        RuntimeKind::Centrifuge => ...,
    }
}

// Inject the tests for all runtimes using the `genesis()` to setup the initial stage.
crate::test_with_all_runtimes!(genesis, test_foo);

will create the following output:

running 3 tests
test test_foo::altair ... ok
test test_foo::centrifuge ... ok
test test_foo::development ... ok

Changes and Descriptions

• Runtime tests framework
• Loan integration tests (with oracles and runtime changes)

[lemunozm]

Close in favor of: #1583