Added test_sink script

To replace my old Sink testing system (GNU Screen and a text file with
the commands to send to the Sink), I wrote a Python script that tests
the Sink automatically.  The only thing I have to do (assuming the test
is successful) is press Enter a few times while watching a voltmeter's
display.  A basic testing script could be useful to other people, so I'm
including it in this Python package.

I have a vague plan to integrate this with Kaylee at some point so I
don't even have to press Enter, but that will have to wait until Kaylee
has menu support.

pdbuddy/tools/test_sink.py

@@ -0,0 +1,97 @@
+"""Test a freshly-flashed PD Buddy Sink"""
+
+import argparse
+import sys
+
+import pdbuddy
+
+
+class ExitCode:
+    configuration_error = 1
+    wrong_output = 2
+
+
+def write_verify(sink, cfg):
+    """Write a SinkConfig to a Sink and verify that it was written"""
+    sink.set_tmpcfg(cfg)
+    sink.write()
+
+    actual_cfg = sink.get_cfg()
+    if actual_cfg != cfg:
+        print('Configuration error; expected config:')
+        print(cfg)
+        print('actual config:')
+        print(actual_cfg)
+        sys.exit(ExitCode.configuration_error)
+
+
+def output_verify(sink):
+    """Verify that the Sink's output is correct"""
+    # Get the configured voltage
+    voltage = sink.get_cfg().v
+    if voltage is None:
+        voltage = 0
+
+    answer = input("Is the Sink's output stable at {:.1f} V [Y,n]? ".format(
+        voltage/1000. * sink.output))
+    if answer.lower().startswith('n'):
+        print('Wrong output, exiting.')
+        sys.exit(ExitCode.wrong_output)
+
+
+def main():
+    # Make the argument parser
+    parser = argparse.ArgumentParser(description='Test a PD Buddy Sink')
+    parser.add_argument('tty', metavar='tty', type=str, help="the Sink's tty")
+
+    # Parse arguments
+    args = parser.parse_args()
+
+    # Define configurations for testing
+    cfg_20v = pdbuddy.SinkConfig(status=pdbuddy.SinkStatus.VALID,
+            flags=pdbuddy.SinkFlags.NONE, v=20000, i=1000)
+    cfg_9v = pdbuddy.SinkConfig(status=pdbuddy.SinkStatus.VALID,
+            flags=pdbuddy.SinkFlags.NONE, v=9000, i=1000)
+
+    # Run the testing procedure
+    with pdbuddy.Sink(args.tty) as sink:
+        # Make sure output is disabled to start
+        sink.output = False
+
+        # Set initial configuration
+        print('Writing 20 V configuration object…')
+        write_verify(sink, cfg_20v)
+        print('Done.')
+
+        # Turn on the output and ensure that it's correct
+        sink.output = True
+        output_verify(sink)
+
+        # Turn off the output and ensure that it's off
+        sink.output = False
+        output_verify(sink)
+
+        # Turn on the output again and ensure that it's back at 20 V
+        sink.output = True
+        output_verify(sink)
+
+        # Set second configuration
+        print('Writing 9 V configuration object…')
+        write_verify(sink, cfg_9v)
+        print('Done.')
+
+        # Ensure that the output is 9 V now
+        output_verify(sink)
+
+        # Erase the sink's configuration
+        sink.erase()
+
+        # Ensure there's no output despite it being enabled
+        sink.output = True
+        output_verify(sink)
+
+    # Congratulations, all tests passed!
+    print('Test successful!')
+
+if __name__ == '__main__':
+    main()