Firmware, Factory Test Bring-up, Add Flow Control Test for COM Port

[ForrestErickson]

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Right now factory tests does not cover the flow control on the COM port.

Describe the solution you'd like

So that the RS232 level shifter components and DB9 connectivity can be verified, the COM test with flow control should be set so that we can measure both the RS232 level shifter output for both MARK and SPACE, (both +6 and -6 volts).

[ForrestErickson]

Test with loopback on RTS to CTS passes

Test with out loopback reports failure as expected.

Ready to pull request!