Increase iteration count for asset tests #29577

Job	Run time
build (windows-latest)	5m 44s
build (ubuntu-latest)	3m 48s
build (macos-latest)	3m 34s
check-bevy-internal-imports	12s
check-doc	9m 17s
check-missing-examples-in-docs	49s
ci	2m 42s
miri	7m 43s
run-examples-on-windows-dx12	15m 56s
check-missing-features-in-docs	42s
markdownlint	1m 24s
check-compiles	4m 13s
msrv	1m 8s
build-wasm	1m 4s
	58m 16s

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