Cleaner merging of Gradle blocks for functionalTests

[TWiStErRob]

Follow-up on #5819 (comment)
Related #5827
Closes #5828

Changes:

• Fix indentation of fixture code (somewhere 4, somewhere 3, somewhere 2 spaces), yaml kept at 2 everything else is 4.
• Replace all remaining trimMargin with trimIndent
• Add some @Language annotations, the build code in functionalTests will be highlighted properly (requires trimMargin removal).
• Re-think how multiple pieces of Gradle script blocks are merged.
  • Previously
    • they were relying on the | being present in the input string,
    • some of them didn't have the leading margin marker.
    • simple string concatenation was done with triple-quotes
    • the generated output was still messy, mis-indented, and hard to follow.
  • In this PR
    • Removed all @Suppress("TrimMultilineRawString")
    • Each code block is self-contained
    • This is achieved by 2 simple rules:
      1. Apply trimIndent() at the definition of the fragment
      2. Deal with indentation where the merging happens
    • This needed two small functions to be introduced: reIndent and mergeGradleBlocks.
      Their usages are pretty localized and make the fragment re-use be at a higher level of abstraction (like """$a $b""" -> merge(a, b)).

You can look at the resulting changes by diffing these two files (note: console output trims empty lines so the effect of \n\n is not clearly visible here, but it is in the JUnit test report):
main.txt
pr.txt

[codecov]

Codecov Report

Merging #5830 (36079c1) into main (059afea) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #5830   +/-   ##
=========================================
  Coverage     84.59%   84.59%           
  Complexity     3790     3790           
=========================================
  Files           546      546           
  Lines         12918    12918           
  Branches       2268     2268           
=========================================
  Hits          10928    10928           
  Misses          861      861           
  Partials       1129     1129           

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[cortinico]

TIL you can do this with "gradle.kts" 👀

[TWiStErRob]

Funny thing: I only learned this 2 days ago while writing this PR too :)

Before that I knew kotlin and gradle were languages, but recently I was having chats about .gradle.kts files as convention plugins (which are way awesome btw), so while I was writing these annotations I just naturally wrote it, and it worked! 😅

[BraisGabin]

👏👏👏 Good one! And the diff of those two files looks WAY WAY better.