OAK-1716 - Enable passing of a execution context to runTest in multi threaded runs

Exposed a protected method `prepareThreadExecutionContext`
which subclasses can override to return a context instance which would
be used by that thread of execution

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1586218 13f79535-47bb-0310-9956-ffa450edef68