Enable automatic test discovery on Linux

[1ec5]

As of apple/swift-package-manager#3053 and Swift 5.4, SPM now supports automatic test discovery on Linux. LinuxMain.swift is now deprecated. We should remove that file to take advantage of automatic test discovery, which will eliminate the need to maintain a list of tests by hand. (Looks like we somehow got away with not having an XCTestManifests.swift?)

Currently the Travis configuration for this repository installs Swift 5.0, so we’ll need to upgrade to 5.4; not sure if it’s supported by swiftenv:

https://github.com/mapbox/turf-swift/blob/62706f39e85ae37b2f7f6e5ac2d38371569266cc/.travis.yml#L7

/ref mapbox/mapbox-directions-swift#539 /cc @mapbox/navigation-ios

[S2Ler]

5.4 isn't supported by swiftenv