Adds generator support for Xcode 5 xctest targets.

Update the xcode generator to set the target productType in the pbxproj file to "com.apple.product-type.bundle.unit-test", if the target sets the "xctest_bundle" property.

Patch by Jon Wall <jonwall@google.com>

BUG=
R=mark@chromium.org

Review URL: https://codereview.chromium.org/51413002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1774 78cadc50-ecff-11dd-a971-7dbc132099af