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.c
	(test_diagnostic_context::test_diagnostic_context): Set
	caret_max_width to 80.

(cherry picked from commit 739e7ebb3d)

gcc/selftest-diagnostic.c

@@ -41,6 +41,7 @@ test_diagnostic_context::test_diagnostic_context ()
   show_column = true;
   start_span = start_span_cb;
   min_margin_width = 6;
+  caret_max_width = 80;
 }
 
 test_diagnostic_context::~test_diagnostic_context ()