Refactor the widget tests to use `MockMatrixServer` #12583

Job	Run time
Generate Crypto FFI Apple XCFramework	5m 51s
Run Complement Crypto tests / tests	9m 15s
xtask / xtask-🐧	4s
xtask / xtask-🍏	7s
Test UniFFI bindings generation	3m 1s
matrix-rust-components-kotlin	5m 52s
matrix-rust-components-swift	6m 41s
	30m 51s

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