timeout function for Futures does not elapse on WASM32

[docweirdo]

After implementing a generic timeout function for futures in matrix-sdk-common with #888, the unit test with_timeout fails for target architecture wasm32.

Instead of using tokio::time::timeout like all other architectures, the timeout function uses the crate wasm_timer for wasm32. After conferring with @poljar, I have disabled the unit test for wasm32, but the issue of course remains.