Note: this does not change the parser behavior at all (except for making
one error message more useful). This is just an internal refactor.

The TAP output parser currently operates over a List[str].
This works, but we only ever need to be able to "peek" at the current
line and the ability to "pop" it off.

Also, using a List means we need to wait for all the output before we
can start parsing. While this is not an issue for most tests which are
really lightweight, we do have some longer (~5 minutes) tests.

This patch introduces an LineStream wrapper class that
* Exposes a peek()/pop() interface instead of manipulating an array
  * this allows us to more easily add debugging code [1]
* Can consume an input from a generator
  * we can now parse results as tests are running (the parser code
  currently doesn't print until the end, so no impact yet).
* Tracks the current line number to print better error messages
* Would allow us to add additional features more easily, e.g. storing
  N previous lines so we can print out invalid lines in context, etc.

[1] The parsing logic is currently quite fragile.
E.g. it'll often say the kernel "CRASHED" if there's something slightly
wrong with the output format. When debugging a test that had some memory
corruption issues, it resulted in very misleading errors from the parser.

Now we could easily add this to trace all the lines consumed and why
+import inspect
...
        def pop(self) -> str:
                n = self._next
+               print(f'popping {n[0]}: {n[1].ljust(40, " ")}| caller={inspect.stack()[1].function}')

Example output:
popping 77: TAP version 14                          | caller=parse_tap_header
popping 78: 1..1                                    | caller=parse_test_plan
popping 79:     # Subtest: kunit_executor_test      | caller=parse_subtest_header
popping 80:     1..2                                | caller=parse_subtest_plan
popping 81:     ok 1 - parse_filter_test            | caller=parse_ok_not_ok_test_case
popping 82:     ok 2 - filter_subsuite_test         | caller=parse_ok_not_ok_test_case
popping 83: ok 1 - kunit_executor_test              | caller=parse_ok_not_ok_test_suite

If we introduce an invalid line, we can see the parser go down the wrong path:
popping 77: TAP version 14                          | caller=parse_tap_header
popping 78: 1..1                                    | caller=parse_test_plan
popping 79:     # Subtest: kunit_executor_test      | caller=parse_subtest_header
popping 80:     1..2                                | caller=parse_subtest_plan
popping 81:     1..2 # this is invalid!             | caller=parse_ok_not_ok_test_case
popping 82:     ok 1 - parse_filter_test            | caller=parse_ok_not_ok_test_case
popping 83:     ok 2 - filter_subsuite_test         | caller=parse_ok_not_ok_test_case
popping 84: ok 1 - kunit_executor_test              | caller=parse_ok_not_ok_test_case
[ERROR] ran out of lines before end token
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>