Rollup merge of #151375 - terminal_width_independence, r=lqd

Fix terminal  width dependent tests

[#t-compiler > What is -Zui-testing=no and why are we using it](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/What.20is.20-Zui-testing.3Dno.20and.20why.20are.20we.20using.20it/with/568842970)

See zulip thread. I've verified locally that this lets me run the ui test suite without problems even with a very thin terminal 😆

Author: Zalathar

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