tests: verify SAN ext. criticality #1746

Job	Run time
Documentation (stable)	1m 13s
Format & clippy	6m 28s
Documentation (nightly)	1m 14s
Build rustls-cert-gen as a standalone package	30s
build (macOS-latest, stable)	3m 55s
Validate external types appearing in public API	1m 39s
build-windows	14m 47s
build (ubuntu-latest, stable)	6m 52s
Measure coverage	6m 50s
build (ubuntu-latest, beta)	6m 50s
Check semver compatibility	1m 1s
build (ubuntu-latest, nightly)	6m 51s
build (ubuntu-latest, stable 7 months ago)	6m 51s
	1h 5m 1s

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