[Tests][Server] WMS GetLegendGraphic JSON for layer with multiple symbols

[rldhont]

Description

Add a test for WMS GetLegendGraphic to JSOn format for a vector layer with multiple symbols

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-45386-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 5f0c7cd7596fe420dd0918340609f7f867ab2dcf
# Push it to GitHub
git push --set-upstream origin backport-45386-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-45386-to-queued_ltr_backports.