Add unit tests target to build script

scripts/build.sh

@@ -3,3 +3,5 @@
 xcodebuild -project SwiftyStoreKit.xcodeproj -target SwiftyStoreKit_iOS
 xcodebuild -project SwiftyStoreKit.xcodeproj -target SwiftyStoreKit_macOS
 xcodebuild -project SwiftyStoreKit.xcodeproj -target SwiftyStoreKit_tvOS
+
+xcodebuild test -project SwiftyStoreKit.xcodeproj -scheme SwiftyStoreKitTests -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone 6,OS=9.3'