tests/lint: fix tests for integer constants on ILP32 platforms

The comment in msg_218 was both off-topic and wrong, so remove it.

lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

tests/lint: fix and extend tests for C90 migration warning

tests/lint: remove redundant tests

The names of the tests were misleading: 'recursive' should have been
'nested', and the interesting topic in the 'cast' tests was not the cast
but the pointer dereference.

lint: add quotes around placeholders for the remaining messages

Reword some of the messages slightly, exchanging brevity for clarity.

Message 138 is kept as-is, as it is not yet covered by any tests.

Message 240 is kep as-is, as it is unreachable.

tests/lint: make 'expect+-' comments stricter

Previously, the expectations from these comments were already satisfied
if the expectation occurred somewhere in the actual message from lint.
This meant that the prefix 'error:' or 'warning:' could be omitted from
the 'expect' comment. These omissions were hard to see in a manual
review. Now any omissions must be visually marked with '...'.

The test msg_342 now reports its messages properly as being in the file
msg_342.c, rather than msg_341.c. This had been a copy-and-paste
mistake.

tests/lint: guard against typos in test environment configuration

Some tests had "lint1-only-if" without a trailing colon, others included
the trailing colon. The tests that included the trailing colon were run
even though they were supposed to be skipped, and they failed, as could
be expected.

To prevent further typos, always require the trailing colon, just as in
"lint1-flags" and fail fast on unknown "lint1" comments.

tests/lint: add test that only runs where char == unsigned char

There a 4 regular NetBSD builds where lint is activated. All these
builds run on platforms where char == signed char.

The official test runs from https://releng.netbsd.org/test-results.html
mostly have char == signed char as well.

However, lint behaves differently on platforms with char == unsigned
char. On these platforms, a simple "char ch = '\xff'" leads to the
bogus warning that "conversion of 'int' to 'char' is out of range".

tests/lint: add tests for ILP32 platforms

Previously, all tests for lint had to produce the exact same output, no
matter which platform they ran on. This differs from practical needs
since lint is intended to produce different results depending on whether
the platform is ILP32 or LP64.

Examples for these are type conversions and the widths of the integer
types during lexical analysis.