selftest: some tests ignore --port spec

[TomKeddie]

This may be all tests, these are the inconsistent results I've observed.

voltage:

(venv) tom@z400:~/git/GlasgowEmbedded/glasgow/software$ venv/bin/glasgow run selftest --port A voltage
I: g.device.hardware: device already has bitstream ID f40ac1bcdf477dc065149f7980d9eaf3
I: g.cli: running handler for applet 'selftest'
I: g.applet.internal.selftest: running self-test mode voltage
E: g.applet.internal.selftest: self-test: FAIL
E: g.applet.internal.selftest: voltage: port B out of ±5% tolerance: Vio=1.80 Vsense=0.00
E: g.applet.internal.selftest: voltage: port B out of ±5% tolerance: Vio=2.70 Vsense=0.00
E: g.applet.internal.selftest: voltage: port B out of ±5% tolerance: Vio=3.30 Vsense=0.00
E: g.applet.internal.selftest: voltage: port B out of ±5% tolerance: Vio=5.00 Vsense=0.00

pins-ext:

(venv) tom@z400:~/git/GlasgowEmbedded/glasgow/software$ venv/bin/glasgow run selftest --port A pins-ext
I: g.device.hardware: device already has bitstream ID f40ac1bcdf477dc065149f7980d9eaf3
I: g.cli: running handler for applet 'selftest'
I: g.applet.internal.selftest: running self-test mode pins-ext
E: g.applet.internal.selftest: self-test: FAIL
E: g.applet.internal.selftest: pins-ext: fail short: B0 B1

[TomKeddie]

This is a dupe of #151