Integrate xcpretty into test target

xcpretty makes Swift test output on the command line much more readable by adding color, spacing, symbols, snippets of failing code, etc, as shown in screenshot below.

although it is not mentioned on the repo, as described here, it works with Swift test output if you run swift test 2>&1 | xcpretty. the 2>&1 is necessary as test output goes to stderr. 

we should add an option to make test (-p or something) that does the necessary redirect and piping.

we should complete SWIFT-103 in tandem with this, as that also involves modifying make test, and without SWIFT-103 our test output is still quite ugly because --filter (which we must use to exclude the benchmarks) places each matching test in a separate suite, leading to very lengthy output.