tools: unittests: add tests for CMatch

The CMatch logic is complex enough to justify tests to ensure
that it is doing its job.

Add unittests to check the functionality provided by CMatch
by replicating expected patterns.

The CMatch class handles with complex macros. Add an unittest
to check if its doing the right thing and detect eventual regressions
as we improve its code.

The initial version was generated using gpt-oss:latest LLM
on my local GPU, as LLMs aren't bad transforming patterns
into unittests.

Yet, the curent version contains only the skeleton of what
LLM produced, as I ended higly changing its content to be
more representative and to have real case scenarios.

The kdoc_xforms test suite contains 3 test groups. Two of
them tests the basic functionality of CMatch to
replace patterns.

The last one (TestRealUsecases) contains real code snippets
from the Kernel with some cleanups to better fit in 80 columns
and uses the same transforms as kernel-doc, thus allowing
to test the logic used inside kdoc_parser to transform
functions, structs and variable patterns.

Its output is like this:

        $ tools/unittests/kdoc_xforms.py
        Ran 25 tests in 0.003s

        OK
	test_cmatch:
	    TestSearch:
	        test_search_acquires_multiple:      OK
	        test_search_acquires_nested_paren:  OK
	        test_search_acquires_simple:        OK
	        test_search_must_hold:              OK
	        test_search_must_hold_shared:       OK
	        test_search_no_false_positive:      OK
	        test_search_no_function:            OK
	        test_search_no_macro_remains:       OK

        Ran 8 tests

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <119712b5bc53b4c6dda6a81b4a783dcbfd1d970d.1773770483.git.mchehab+huawei@kernel.org>

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