helpful error message when compiling without test discovery on >= 5.5

[weissi]

Motivation:

When compiling on Swift >= 5.5, we require --enable-test-discovery to be passed to swift build --build-tests/swift test. But the error message was terrible.

Modification:

Give a helpful error message when compiling without --enable-test-discovery.

Result:

Less puzzled users.

[Lukasa]

Thanks!