run_net_test.sh: switch to readonly by default

And also add support for --readwrite --rw --ro flags.

Read only by default is better cause it allows multiple parallel
invocations making use of the same image.

Read write access to the file system is only occasionally useful,
and can always be requested by hand.

While I'm at it, change some occurences of [ to [[ to honour
Google's shell style guide:
  https://google.github.io/styleguide/shell.xml.

Change-Id: I94ac0d9d05470bfd5e62465454c398174d644e5b