test

Add tests for `JWTError` and `JWTHeader+CommonFields` (#188) #531

Job	Run time
linux-integration	3m 28s
ios-tests	4m 34s
unit-tests / api-breakage	0s
unit-tests / linux-unit (swift:5.8-jammy)	26s
unit-tests / linux-unit (swift:5.9-jammy)	26s
unit-tests / linux-unit (swift:5.10-noble)	5m 47s
unit-tests / linux-unit (swiftlang/swift:nightly-6.0-jammy)	5m 29s
unit-tests / linux-unit (swiftlang/swift:nightly-main-jammy)	6m 19s
unit-tests / macos-unit (macos-13, ~15.2)	16s
unit-tests / macos-unit (macos-14, latest-stable)	6m 29s
	33m 14s

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