Adds CI configuration to run web integration tests (in the master channel) compiled to Wasm.

It also removes the `build-examples` step from web integration tests, in some isolated testing:

| platform tests shard | With build-examples | Without build-examples |
|---|-----|-----|
| 1 | 30m | 21m |
| 2 | 13m | 11m |
| 3 | 17m | 10m |

## Issues

* Fixes https://github.com/flutter/flutter/issues/151664