kunit: tool: refactoring printing logic into kunit_printer.py

Context:
* kunit_kernel.py is importing kunit_parser.py just to use the
  print_with_timestamp() function
* the parser is directly printing to stdout, which will become an issue
  if we ever try to run multiple kernels in parallel

This patch introduces a kunit_printer.py file and migrates callers of
kunit_parser.print_with_timestamp() to call
kunit_printer.stdout.print_with_timestamp() instead.

Future changes:
If we want to support showing results for parallel runs, we could then
create new Printer's that don't directly write to stdout and refactor
the code to pass around these Printer objects.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>