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.

branches: 1.1.2;
lint: add more specific warning for bit-field of type plain 'int'

Previously, declaring a bit-field of type plain 'int' resulted in this
warning:

warning: nonportable bit-field type 'int' [34]

This warning was too unspecific to be actionable, and until yesterday it
didn't even include the type. In order to allow this warning to be
understood and properly fixed, describe the actual nonportability more
precisely:

warning: bit-field of type plain 'int' has
implementation-defined signedness [344]