Support Multi-Thread Tests

[adamj537]

Feature
We need to support bed-of-nails fixtures with one or more devices under test. For fixtures with more than one device under test, the

Solution
Modify Sensit.App.Pogo to have a persistent setting for number of devices under test. We can assume that for each device under test, there will be separate test equipment, so we also need persistent settings for each set of test equipment (e.g., separate serial ports or device addresses).

When a test is started, the app should create a separate thread for each device under test, and execute tests at the same time. The SDK supports some devices that can be daisy-chained on a single serial port via RS-485 or RS-232, so shared resources such as serial ports must be managed between threads.