tests: verify SAN ext. criticality #1748

Job	Run time
Check semver compatibility	1m 6s
build-windows	13m 3s
Validate external types appearing in public API	1m 45s
Format & clippy	6m 32s
Documentation (stable)	1m 13s
Build rustls-cert-gen as a standalone package	32s
build (macOS-latest, stable)	3m 41s
build (ubuntu-latest, stable)	6m 46s
Documentation (nightly)	1m 8s
build (ubuntu-latest, beta)	6m 42s
build (ubuntu-latest, nightly)	6m 39s
Measure coverage	6m 56s
build (ubuntu-latest, stable 7 months ago)	6m 43s
	1h 2m 46s

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