Updated run_tests.sh to run tests in debug mode

Start directory was not configured property in run_tests.sh which raises
error while running entire test suit using debug mode.

Updated "testrargs" value from "discover ./glance/tests" to
"discover ./tests" as default starting point to run all tests
if specific test is not provided.

Closes-Bug: 1383117
Change-Id: Ie26d6ec02c92e9ac03dd7c35d313ea377715322f