doc: add a guide for developing unit tests

doc: add a guide for developing unit tests

The DPDK testing infrastructure includes a comprehensive set of
libraries, utilities, and CI integrations for developers to test
their code changes.  This isn't well documented, however.

Document the basics for adding a test suite to the infrastructure
and enabling that test suite for continuous integration platforms
so that newer developers can understand how to develop test suites
and test cases.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Ciara Power <ciara.power@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>