Move UI example to testbed #52077

Job	Run time
miri	10m 44s
toml	2m 1s
build (windows-latest)	10m 44s
ci	4m 41s
build (ubuntu-latest)	4m 33s
typos	8s
build (macos-latest)	3m 16s
check-bevy-internal-imports	5s
check-doc	12m 8s
check-missing-examples-in-docs	34s
run-examples-macos-metal	6m 31s
check-missing-features-in-docs	28s
markdownlint	1m 9s
check-compiles-no-std	52s
check-compiles	11m 30s
msrv	1m 59s
build-wasm-atomics	1m 57s
build-wasm	1m 49s
	1h 15m 9s

Provide feedback