compiletest: ignore SVG y offset in by-lines comparison

[xmakro]

Tracked in #154314.

SVG diagnostic snapshots render one terminal row per line, laid out top to bottom. Under the parallel front-end, independent diagnostics can be emitted in a different order. That permutes the rows and re-renders each one at a different vertical y offset, even though the content of every row is unchanged.

This change strips the y coordinate (and the <svg> header line, whose width and height are already ignored by the exact comparison) before the multiset comparison, so a reordering of whole rows is no longer treated as a difference.

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

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[petrochenkov]

@zetanumbers
At least the root cause analyses look plausible (although they need to be verified by a human of course), and may be potentially put into the tracking issue, even if the change is not merged.

View changes since the review

[xmakro]

When running the SVG test under the parallel frontend at head, then any run fails the existing line by line comparison, and compiletest writes the actual rendered SVG to disk:

RS=tests/ui/error-emitter/multiline-removal-suggestion.rs
for i in $(seq 1 40); do
  ./x test $RS --test-args "--parallel-frontend-threads=16" --force-rerun
done
# each failing run prints: Saved the actual svg to `<build>/.../multiline-removal-suggestion.svg`

Raw sorted line comparison, which is what the test does before this PR:

$ diff <(sort A.svg) <(sort B.svg) | grep -c '^[<>]'
232      # nonzero, so the comparison reports a difference

By comparison, with this PR, we drop the <svg> header line, replace y="..px" with a constant, then sort:

$ norm(){ tail -n +2 "$1" | sed -E 's/y="[0-9]+px"/y="0px"/g' | sort; }
$ diff <(norm A.svg) <(norm B.svg) | grep -c '^[<>]'
0        # identical as a multiset

Each differing line is the same row at a different height. For example the rows for the span at :39:2:

A: <tspan x="10px" y="1648px"> ... rs:39:2</tspan>
B: <tspan x="10px" y="370px"> ... rs:39:2</tspan>

The content is the same but the y changes. After the substitution both become y="0px" and match.

[zetanumbers]

This change should also be reflected in diff_by_lines function for rendering the line difference.

[zetanumbers]

We can confirm it works if test passes now, but it seems to be about right.

UPDATE: I've checked it works.

[xmakro]

Done

[zetanumbers]

А ты говоришь на русском?