Run tests on watchOS

[1ec5]

Xcode 12 supports watchOS test targets, so we should add a test target to the Xcode project for watchOS to ensure code coverage.

If #495 lands before this issue is fixed, then we’ll need to update AliSoftware/OHHTTPStubs#286 on a separate fork.

/cc @mapbox/navigation-ios @frederoni

[1ec5]

watchOS doesn’t support XCTest, but XCTest-watchOS is a third-party implementation that supports both Carthage and SPM.