Fix terminal dependent tests

Author: BoxyUwU

src/tests/ui.md

@@ -139,6 +139,10 @@ prefixing each source line are replaced with `LL`). In extremely rare
 situations, this mode can be disabled with the directive `//@
 compile-flags: -Z ui-testing=no`.
 
+When using `-Z ui-testing=no` the `--diagnostic-width` argument should also
+be set to avoid tests failing or passing depending on the width of the terminal
+from which the UI test suite is being run.
+
 Note: The line and column numbers for `-->` lines pointing to the test are *not*
 normalized, and left as-is. This ensures that the compiler continues to point to
 the correct location, and keeps the stderr files readable. Ideally all