Use testr for running gate tests

Fixes bug 1172468

Change-Id: Ifd87e44d752b4b3c77b4ea8f38feceb03bccaebe
Remove examples and update testing-overview.txt

With the migration to testrepository, many of the instructions are
obsolete. Also the examples are not needed as we have plenty of fine
examples available in the code itself.

Change-Id: I87524eaab4c39ea75d7e53fb11ffe103d5339cc3