Upgrade coroutines to 1.6.4 (including coroutines.test changes)

[steve-the-edwards]

For non-test use we had already migrated from offer to trySend for channels.

For tests (the larger change), see the test migration guide followed here: https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-test/MIGRATION.md

This has been tested successfully internally in Square POS via a snapshot.

Regarding whether we should convert all runBlocking to runTest when used in tests, its a choice for each situation (we have converted many but not all). See Dmitry Khalanskiy's (coroutines maintainer) to that question:

If you're not relying on virtual time, you can stick with runBlocking, it's just a bit less convenient:
There's no runBlocking on JS, so the test won't be usable in multiplatform code. This was the message behind the line: 1.6 introduced multiplatform coroutine testing.
runBlocking returns the value that its block returns. JUnit, for example, expects the test functions to return Unit. So, something like @test fun test() = runBlocking { assertFailsWith { throw IllegalArgumentException() } } will, surprisingly, not be executed by JUnit, because test() returns an IllegalArgumentException.
runTest is purely for tests, which means we can enhance it with other niceties that have no place in production code. For example, we recently added a way to cancel some of the coroutines in a test when the test body finishes: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-scope/background-scope.html You won't have that with runBlocking.

[vgonda]

Sweet! Learn stuff we can apply to register? 😉