WidgetDriver: Use matrix_sdk_common::executor::spawn instead of tokio::spawn to make it wasm compatible.

[toger5]

This PR is a revived version of: #4707 since it is now possible to easily add wasm support thanks to: #4572

Using the executer::spawn will select tokio::spawn or wasm_bindgen_futures::spawn_local based on what platform we are on.

They behave differently in that tokio actually starts the async block inside the spawn but the wasm spawn wrapper will only start the part that is not inside the async block and the join handle needs to be awaited for it to work.

This has now changed with: #4572.
Now they behave the same and this PR becomes a very simple change.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (d2874af) to head (ae83b6c).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4959      +/-   ##
==========================================
- Coverage   85.95%   85.93%   -0.02%     
==========================================
  Files         325      325              
  Lines       35649    35649              
==========================================
- Hits        30641    30636       -5     
- Misses       5008     5013       +5     

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[bnjbvr]

Thanks! just one small nit :)