lint: fix wording of diagnostics about 'C99 extension'

The term 'extension' means an extension to a C standard. C99 by itself
is not an extension but a revision of the C standard.

tests/lint: add more details to tests from msg_300 until msg_343

lint: force each test to declare the expected diagnostics

By listing the expected diagnostics directly at the code that triggers
the diagnostics, it is easier to cross-check whether the diagnostics

lint: force each test to declare the expected diagnostics

By listing the expected diagnostics directly at the code that triggers
the diagnostics, it is easier to cross-check whether the diagnostics
make sense.

No functional change to lint itself.

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lint: add a test for each message produced by lint1

Having a test for each message ensures that upcoming refactorings don't
break the basic functionality. Adding the tests will also discover
previo

lint: add a test for each message produced by lint1

Having a test for each message ensures that upcoming refactorings don't
break the basic functionality. Adding the tests will also discover
previously unknown bugs in lint.

The tests ensure that every lint message can actually be triggered, and
they demonstrate how to do so. Having a separate file for each test
leaves enough space for documenting historical anecdotes, rationale or
edge cases, keeping them away from the source code.

The interesting details of this commit are in Makefile and
t_integration.sh. All other files are just auto-generated.

When running the tests as part of ATF, they are packed together as a
single test case. Conceptually, it would have been better to have each
test as a separate test case, but ATF quickly becomes very slow as soon
as a test program defines too many test cases, and 50 is already too
many. The time complexity is O(n^2), not O(n) as one would expect.
It's the same problem as in tests/usr.bin/make, which has over 300 test
cases as well.

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