Add libSelenium static library target

Attempting to link against Selenium.framework from within an XCTest bundle (and not from within a target app) creates runpath errors on test execution, due to dyld searching for Selenium.framework at @executable_path/../Frameworks/Selenium.framework, where @executable_path is the target app and not the XCTest bundle.

To work around this, I made a new target for the existing code to build a static library called libSelenium, which does link and run successfully from within an XCTest bundle.