[build-script] Build & test XCTest (on OS X, too)

Allow swift-corelibs-xctest to be built and tested on Linux and OS X,
via `utils/build-script --xctest --test`.

On OS X, XCTest is built via `xcodebuild`, which has been possible since
swiftlang/swift-corelibs-xctest#47. It's tested via
the "SwiftXCTestFunctionalTests" Xcode target. Keep in mind that
`xcodebuild` must be configured on the host machine to use a Swift
toolchain that can build XCTest--as of
swiftlang/swift-corelibs-xctest#48, that would be
`swift-DEVELOPMENT-SNAPSHOT-2016-02-08` or later.

On Linux, XCTest is built and tested via the project's
`build_script.py`, which has been possible since
swiftlang/swift-corelibs-xctest#46.

Author: modocache

utils/build-script-impl

@@ -1746,11 +1746,17 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                 continue
                 ;;
             xctest)
-                SWIFTC_BIN="$(build_directory_bin ${deployment_target} swift)/swiftc"
-                SWIFT_BUILD_PATH="$(build_directory ${deployment_target} swift)"
-                set -x
-                "${XCTEST_SOURCE_DIR}"/build_script.py --swiftc="${SWIFTC_BIN}" --build-dir="${build_dir}" --swift-build-dir="${SWIFT_BUILD_PATH}" --arch="${SWIFT_HOST_VARIANT_ARCH}"
-                { set +x; } 2>/dev/null
+                if [[ "$(uname -s)" == "Darwin" ]] ; then
+                    set -x
+                    xcodebuild -project "${XCTEST_SOURCE_DIR}"/XCTest.xcodeproj -scheme SwiftXCTest SKIP_INSTALL=NO DEPLOYMENT_LOCATION=YES DSTROOT="${build_dir}" INSTALL_PATH="/"
+                    { set +x; } 2>/dev/null
+                else
+                    SWIFTC_BIN="$(build_directory_bin ${deployment_target} swift)/swiftc"
+                    SWIFT_BUILD_PATH="$(build_directory ${deployment_target} swift)"
+                    set -x
+                    "${XCTEST_SOURCE_DIR}"/build_script.py --swiftc="${SWIFTC_BIN}" --build-dir="${build_dir}" --swift-build-dir="${SWIFT_BUILD_PATH}" --arch="${SWIFT_HOST_VARIANT_ARCH}"
+                    { set +x; } 2>/dev/null
+                fi
 
                 # XCTest builds itself and doesn't rely on cmake
                 continue
@@ -1991,7 +1997,21 @@ for deployment_target in "${STDLIB_DEPLOYMENT_TARGETS[@]}"; do
                 if [[ "${SKIP_TEST_XCTEST}" ]]; then
                     continue
                 fi
-                # FIXME: We don't test xctest, yet...
+                echo "--- Running tests for ${product} ---"
+                if [[ "$(uname -s)" == "Darwin" ]] ; then
+                    set -x
+                    xcodebuild -project "${XCTEST_SOURCE_DIR}"/XCTest.xcodeproj -scheme SwiftXCTestFunctionalTests
+                    { set +x; } 2>/dev/null
+                else
+                    SWIFTC_BIN="$(build_directory_bin ${deployment_target} swift)/swiftc"
+                    SWIFT_BUILD_PATH="$(build_directory ${deployment_target} swift)"
+                    set -x
+                    # FIXME: This re-builds swift-corelibs-xctest. Instead, 'build_script.py' should take
+                    # a top-level 'test' command that only tests an already built XCTest.
+                    "${XCTEST_SOURCE_DIR}"/build_script.py --swiftc="${SWIFTC_BIN}" --build-dir="${build_dir}" --swift-build-dir="${SWIFT_BUILD_PATH}" --arch="${SWIFT_HOST_VARIANT_ARCH}" --test
+                    { set +x; } 2>/dev/null
+                fi
+                echo "--- Finished tests for ${product} ---"
                 continue
                 ;;
             foundation)