lint: warn about extern declarations outside headers

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

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: test parsing of octal integer constants

tests/lint: test parsing of integer constants

The previous version of this test did not focus on the integer constants
but instead on conversions of function arguments. The current test
covers several corner cases, such as non-decimal bases and all
combinations of suffixes.

This test does not cover lex_integer_constant completely since several
code paths are only reachable on 32-bit target platforms.

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: test binary integer literals and underscores

tests/lint: test lexing of integer suffixes

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: allow tests to be skipped depending on platform properties

tests/lint: allow skipping individual tests

Depending on the platform, some tests do not make sense or produce
platform-dependent results. Allow these tests to be marked as such.

For example, the test lex_integer.c only works on 64-bit platforms.
Therefore it is disabled on i386 for now since it prints different
warnings there. Even better would be a "lint1-only-on-lpi32" toggle,
but that would need detection of 'sizeof(int)' at runtime.

tests/lint: add test cases for lexical analysis