With the parser rework [1] and run_kernel() rework [2], this allows the
parser to print out test results incrementally.

Currently, that's held up by the fact that the LineStream eagerly
pre-fetches the next line when you call pop().
This blocks parse_test_result() from returning until the line *after*
the "ok 1 - test name" line is also printed.

One can see this with the following example:
$ (echo -e 'TAP version 14\n1..3\nok 1 - fake test'; sleep 2; echo -e 'ok 2 - fake test 2'; sleep 3; echo -e 'ok 3 - fake test 3') | ./tools/testing/kunit/kunit.py parse

Before this patch [1]: there's a pause before 'fake test' is printed.
After this patch: 'fake test' is printed out immediately.

This patch also adds
* a unit test to verify LineStream's behavior directly
* a test case to ensure that it's lazily calling the generator
* an explicit exception for when users go beyond EOF

[1] https://lore.kernel.org/linux-kselftest/20211006170049.106852-1-dlatypov@google.com/
[2] https://lore.kernel.org/linux-kselftest/20211005011340.2826268-1-dlatypov@google.com/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>