Fix tests to match behaviour expected with newest toolchain

[artemcm]

Newest toolchains contain the swift --> swift-frontend rename so a couple of tests needed fixing up.

[artemcm]

@swift-ci please test

[DougGregor]

Excellent, thank you! Can you fix up the README as well?

[artemcm]

@swift-ci please test

[JhonnyBillM]

Hello, when testing in my machine I get could not decode frontend target info; compiler driver and frontend executables may be incompatible.
I have been testing in macOS 10.15.6 with Xcode 11.6 (11E708), Xcode 12.0 beta 6 (12A8189n) and the latest Swift 5.3 Snapshot, in any of those scenarios I have a swift-frontend as mentioned in the README; I am a little bit confused by this, any idea on what is the correct set up for the Driver to find the correct target info?

Cc @owenv since you worked on the error handling/reporting

Thanks in advance!

[owenv]

@JhonnyBillM right now, swift-driver requires a trunk development snapshot from swift.org, the 5.3 snapshots are missing some more recent required changes. I'd try updating that first, and I'll add a note to the README to clarify that requirement. If that isn't enough to get things working, do you mind posting in the Development sections of the forums and we can debug more there?