Add unittest for 'AR' in 'make_global_settings'

Currently sharing the code among host/NaCl/NDK/Emscripten on top of Ninja+gyp isn't so easy as it should be because not all items in 'make_global_settings' are recognized by Ninja generator (gyp:434).

On the other hand, the current test coverage of 'make_global_settings' isn't so good. Thus this CL describes the current behavior with some unittests before making Ninja generator aware of 'AR' in 'make_global_settings' like Make generator.

BUG=gyp:434
TEST=unittest
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/320743002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1931 78cadc50-ecff-11dd-a971-7dbc132099af