Add retry mechanism for rustdoc GUI tests to reduce flakyness

[GuillaumeGomez]

Part of #93784.

I added 3 retries for failing GUI tests. An important note: if more than half of total tests fail, I don't retry because it's very likely not flakyness anymore at this point but a missing update after changes.

[rustbot]

r? @Mark-Simulacrum

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[GuillaumeGomez]

I added some documentation on the NB_RETRY constant too.

[Mark-Simulacrum]

@bors r+ rollup=iffy

[bors]

📌 Commit f902200 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit f902200 with merge cc47b06...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing cc47b06 to master...

[rust-timer]

Finished benchmarking commit (cc47b06): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.5%, 1.1%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.5%	[-1.7%, -1.3%]	2
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.