Update jsondocck directives to follow ui_test-style

[fmease]

Context: Comment chain in #125813.
Follow-up to #126788.
Use the same temporary approach of "double parsing" until we figure out how we want to support compiletest/ui_test directive "add-ons" for child test runners like HtmlDocCk and JsonDocCk.

I didn't touch much of jsondocck because I want to refactor it some other time (for robustness, maintainability and better diagnostics; basically by following a similar design of my WIP HtmlDocCk-next, cc #125780).

r? @GuillaumeGomez

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[GuillaumeGomez]

Nice work, thanks! Since it's conflict prone, gonna up the priority a bit.

@bors r+ p=1

[bors]

📌 Commit 633f41d has been approved by GuillaumeGomez

It is now in the queue for this repository.

[bors]

⌛ Testing commit 633f41d with merge 9057c3f...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 9057c3f to master...

[rust-timer]

Finished benchmarking commit (9057c3f): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary 5.1%)

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)	5.1%	[5.1%, 5.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 767.644s -> 770.598s (0.38%)
Artifact size: 328.87 MiB -> 328.83 MiB (-0.01%)