check-sr-results: combine num eth informations

We can get the number of expected ethernet interfaces from two sources now:

- From the command line with the `--ethernet-devices' option.
- From the `report.txt' with the `Total number of network controllers'
  field.

Combine those two sources, with command line taking precedence.
Add a unit test.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>