Fix #2: "ReadableStreamType.readSome crashes with error "6555 Illegal instruction: 4" when using a Release build of SwiftShell."

Also fixes #1: "SwiftShell_Speed_Tests unit tests never finish.".
It turns out NSString(data data: NSData, encoding encoding: UInt) returns an optional String, something Xcode/Swift neglected to tell me. Which is strange as they are normally very strict about these things. But what really solved these 2 issues (which are really the same issue) was removing the unnecessary cast "as String". For more on this really bizarre bug in the Swift compiler, see https://devforums.apple.com/message/1084852#1084852 .
Skip speed test "testSpeedSwiftSplit" as it takes a very long time.

Also improve output of "testWhenSplitFileAsStringBecomesQuicker".