Issue 62 - Prepare SPI tester framework

* Create SPI tester framework
 * new module spi-tester
 * new source set for spi test code
* Setup example framework use for Store SPI

This does not complete issue #62, only prepares the next step for it.
The only place this generic type is referenced is in the protected variable which is not accessed from here.  Could we not eliminate this, and then let the subclass handle the method by which the testable unit is created?