Also removed a spurious `print()` that got left in by accident,
and disabled the `CodableBacktrace.swift` test on Linux since we
need Foundation and we don't have it there.
rdar://124913332
`JSONEncoder` by default will escape slashes, which results in a string
that isn't technically valid Base64. Since that behaviour is optional,
turn it off, and at the same time tell it to output in lexical key
order, which makes the test slightly simpler (no `CHECK-DAG` needed).
Also fixed a typo in `test_swift.py`
rdar://124913332
Alastair Houghton