kunit: tool: use dataclass instead of collections.namedtuple

namedtuple is a terse way of defining a collection of fields.
However, it does not allow us to annotate the type of these fields.
It also doesn't let us have any sort of inheritance between types.

Since commit df4b0807 ("kunit: tool: Assert the version
requirement"), kunit.py has asserted that it's running on python >=3.7.

So in that case use a 3.7 feature, dataclasses, to replace these.

Changes in detail:
* Make KunitExecRequest contain all the fields needed for exec_tests
* Use inheritance to dedupe fields
  * also allows us to e.g. pass a KUnitRequest in as a KUnitParseRequest
  * this has changed around the order of some fields
* Use named arguments when constructing all request objects in kunit.py
  * This is to prevent accidentally mixing up fields, etc.

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