test: Subtract code_offset from width for ui_testing
`annotate-snippets` does not have a "UI test" mode like `rustc`, [where the code offset is not subtracted from the column width](f34ba774c7/compiler/rustc_errors/src/emitter.rs (L1985-L1987)). This makes it so `annotate-snippets` will shift the output for some very long tests 5 - 7 columns to the left. As part of my work to have `rustc` use `annotate-snippets`, and to reduce the test differences between the two, I figured it would be best if `rustc` started subtracting the code offset from the width as well.
The first commit exists to keep the test output changes of adding a new line to a test separate from adding the `--diagnostic-width` flag in the second commit. This makes it easier to verify that adding the flag does not affect the test's output.
[Zulip discussion](https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/annotate-snippets.20hurdles)
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