[stdlib] Make test for Sequence.map actually test a sequence

[ahoppen]

AnySequence just forwards the call to map to its underlying storage which currently is an Array (and thus a collection).
Using MinimalSequence guarantees that we are actually calling a Sequence.map.

[ahoppen]

@swift-ci Please smoke test

[airspeedswift]

stride might, at some point, become a collection. There are dedicated types in the std lib test suite for when you explicitly want to test a particular protocol – have a look for uses of MinimalSequence.

[ahoppen]

Updated to use MinimalSequence.

@swift-ci Please smoke test

[airspeedswift]

LGTM!

[rudkx]

@ahoppen StdlibCollectionUnittest is only built for validation tests, so this will fail for anyone who runs the plain test suite locally.