Fix crash when querying a runtime config that is defined in environment

Fix crash when querying a runtime config that is defined in environment

Fix crash when querying a runtime config in case when there's no
skia.conf file and the config variable is still defined in the
environment.

Runs the added SkRTConf::UnitTest test as part of new "UnitTest" test.

Previous version of the patch failed Windows build due to setenv usage.
On Windows, use _putenv_s instead.

BUG=skia:1494
R=bsalomon@google.com, humper@google.com

Author: kkinnunen@nvidia.com

Review URL: https://chromiumcodereview.appspot.com/23174002

git-svn-id: http://skia.googlecode.com/svn/trunk@10715 2bbb7eff-a529-9590-31e7-b0007b416f81