[presets] Add swift package tests to the OS X package builders.

Author: Mishal Awadah

utils/build-presets.ini

@@ -557,6 +557,10 @@ install-symroot=%(install_symroot)s
 # installed.
 install-prefix=%(install_toolchain_dir)s/usr
 
+# Executes the lit tests for the installable package that is created
+# Assumes the swift-package-tests repo is checked out
+test-installable-package=1
+
 # If someone uses this for incremental builds, force reconfiguration.
 reconfigure