Alternative paths can be specified for browsers in test-runner

Each browser now has a list of potential paths to check instead of a
single path. This makes it possible to include multiple common install
locations for each browser and reduces the need for every user to do
their own configuration.
Add possible Linux install location of Chromium for test-runner

Chromium is installed to /usr/bin/chromium-browser by the package manager
in at least Ubuntu 12.04.
Merge pull request #419 from NVIDIA/test-runner-improvements

Test runner improvements