Add API for awaiting asynchronous Compose resource loads

[Konstantin (terrakok)]

Add the experimental awaitPendingResourceStateLoads() API, which allows tests to wait until all currently pending asynchronous resource loads have completed and their results have been written to Compose states.

On blocking platforms the function is a no-op because resource states are calculated synchronously.

Web resource APIs such as stringResource, painterResource, and Font return a default value during the initial composition and update their Compose state after the resource has been loaded asynchronously.

waitForIdle() only waits for Compose work. It does not guarantee that resource loading running outside of Compose has completed. As a result, tests could observe placeholders or stale values even after the UI was considered idle.

The required test sequence is:

waitForIdle()                      // perform composition and start resource loads
awaitPendingResourceStateLoads()   // wait until loaded values are written to State
waitForIdle()                      // process recomposition, layout, and draw

Fixes https://youtrack.jetbrains.com/issue/CMP-8776

Release Notes

Features - Resources

• New API for awaiting asynchronous Compose resource loads: awaitPendingResourceStateLoads()