diagnostics: Fix up selftests with $COLUMNS < 42 [PR108973]

diagnostics: Fix up selftests with $COLUMNS < 42 [PR108973]

As mentioned in the PR, GCC's diagnostics self-tests fail if $COLUMNS < 42.
Guarding each self-test with if (get_terminal_width () > 41) or similar
would be a maintainance nightmare (PR has a patch to do so without
reformatting to make it work for $COLUMNS in [30, 41] inclusive, but
I'm afraid going down to $COLUMNS 1 would mean marking everything).
Furthermore, the self-tests don't really emit stuff to the terminal,
but into a buffer, so using get_terminal_width () for it seems
inappropriate.  The following patch makes sure test_diagnostic_context
constructor uses exactly 80 columns wide caret max width, of course
some tests override it already if they want to test for behavior in narrower
cases.

2023-03-04  Jakub Jelinek  <jakub@redhat.com>

	PR testsuite/108973
	* selftest-diagnostic.cc
	(test_diagnostic_context::test_diagnostic_context): Set
	caret_max_width to 80.

(cherry picked from commit 739e7ebb3d378ece25d64b39baae47c584253498)