`apt-get install` commands were present when the nightly build was introduced. Prior to 12b99f0 they were in a separate section without `set -eu -o pipefail`, so the error was ignored. I assume they are not needed, since the build was succeeding despite the error.
While here, ensure xcpretty is installed.
Broke up the iOS and macOS Bitrise scripts into several steps each for more granular progress and failure reporting and to take advantage of better error reporting in the built-in Bitrise steps. Added a step to deploy Xcode test results and (in the event of a failure) raw xcodebuild output to Bitrise.io, with a handy link from the Bitrise UI.
