test: make sure that marked-failing synthetics actually fail #838

Job	Run time
changes	6s
docs / gh-pages	19s
java / test-java (windows-latest)	5m 14s
ts / test-js	53s
java / test-java (macos-latest)	3m 4s
cpp / cpp-build (macos-15)	9m 18s
rust / test	4m 12s
java / test-java (ubuntu-latest)	3m 32s
rust / check	1m 7s
rust / coverage	5m 14s
rust / Test wasm	1m 39s
rust / Test workspace publishing using --dry-run	1m 47s
rust / Test mlt-py	1m 3s
rust / Build release binaries for x86_64-unknown-linux-gnu	1m 9s
rust / Build @maplibre/mlt-wasm npm package	1m 34s
rust / lint	1m 13s
rust / Fuzz mvt_roundtrip	4m 38s
rust / Fuzz layer	5m 37s
rust / Fuzz decoded_layer	4m 14s
rust / FFI cpp	29s
rust / FFI c	36s
rust / Build release binaries for x86_64-pc-windows-msvc	2m 27s
rust / Build release binaries for aarch64-apple-darwin	1m 1s
rust / Build release binaries for aarch64-unknown-linux-gnu	1m 0s
cpp / cpp-build (ubuntu-24.04)	5m 44s
rust / FFI kotlin	1m 2s
rust / ci-passed-rust	2s
ci-passed	3s
rust / release-plz	-1s
rust / Publish @maplibre/mlt-wasm to npm	-1s
rust / Upload release binaries to GitHub releases	-1s
	1h 8m 14s

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