Fix OS_TEST_PATH in .testr.conf

As of commit 87f772c0bd59790aa43de59938d9c138963f9f4e the default
value for OS_TEST_PATH as set in .testr.conf is no longer valid.

In addition, because shell variables are evaluated in a
somewhat unintuitive order in situations like this, the default
wasn't being used properly anyway.  See link below for an
explanation why:

http://stackoverflow.com/questions/10938483/bash-specifying-environment-variables-for-echo-on-command-line

This commit fixes both problems.

Change-Id: I8eec146a21eab656cb48b9e23a92a9da52246269
Merge "Fix OS_TEST_PATH in .testr.conf"