docs: unittests: add a parser to test kernel-doc parser logic

docs: unittests: add a parser to test kernel-doc parser logic

Validating that kernel-doc is parsing data properly is tricky.

Add an unittest skeleton that alllows passing a source code
and check if the corresponding values of export_table and
entries returned by the parser are properly filled.

It works by mocking a file input with the contents of a source
string, an comparing if:

	- exports set matches;
	- expected KernelItem entries match.

Create a new TestSelfValidate meant to check if the logic
inside KdocParser.run_test() does its job of checking for
differences inside KdocItem.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <8d91bfabd69de7aa44a0f5080ccb01aa41957e6d.1773823995.git.mchehab+huawei@kernel.org>