kunit: tool: fix unintentional statefulness in run_kernel()

This is a bug that has been present since the first version of this
code.
Using [] as a default parameter is dangerous, since it's mutable.

Example using the REPL:
>>> def bad(param = []):
...     param.append(len(param))
...     print(param)
...
>>> bad()
[0]
>>> bad()
[0, 1]

This wasn't a concern in the past since it would just keep appending the
same values to it.

E.g. before, `args` would just grow in size like:
  [mem=1G', 'console=tty']
  [mem=1G', 'console=tty', mem=1G', 'console=tty']

But with now filter_glob, this is more dangerous, e.g.
  run_kernel(filter_glob='my-test*') # default modified here
  run_kernel()			     # filter_glob still applies here!
That earlier `filter_glob` will affect all subsequent calls that don't
specify `args`.

Note: currently the kunit tool only calls run_kernel() at most once, so
it's not possible to trigger any negative side-effects right now.

Fixes: 6ebf5866

 ("kunit: tool: add Python wrappers for running KUnit tests")
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>