Fix gyptest-type-envvars.py for Xcode 5.0.0

With Xcode 5.0.0, the SDKROOT environment variable is always set
when building, even if not set in the Xcode project.

BUG=381
R=mark@chromium.org

Review URL: https://codereview.chromium.org/82763006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1863 78cadc50-ecff-11dd-a971-7dbc132099af
Remove explicit usage of sh to launch test_check_sdkroot.sh

The property svn:executable has been set on the test_check_sdkroot.sh script
so remove the workaround that was introduced to have tests pass when the
property was not set.

BUG=None
R=mark@chromium.org

Review URL: https://codereview.chromium.org/190953006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1864 78cadc50-ecff-11dd-a971-7dbc132099af