test: add visual snapshots and comparison

[leanmendoza]

This PR introduces a method in the ~system/Testing API to generate a visual snapshot of the Scene and compare it with the existing one (if it applies).
The current method for comparison is to compute the grayscale from the NTSC standard of each image (the stored snapshot and the just taken one), calculate the absolute difference, and then compute a standard deviation.
The similarity is 1.0, completely similar to 0.0, completely different.

branch test/snapshot
https://github.com/sdilauro/dae-unit-tests

[codecov-commenter]

Codecov Report

Attention: 261 lines in your changes are missing coverage. Please review.

Comparison is base (c854e25) 13.03% compared to head (e8495c7) 12.82%.

Files	Patch %	Lines
rust/decentraland-godot-lib/src/dcl/js/testing.rs	0.00%	83 Missing ⚠️
...raland-godot-lib/src/scene_runner/scene_manager.rs	0.00%	53 Missing ⚠️
...traland-godot-lib/src/test_runner/testing_tools.rs	4.54%	42 Missing ⚠️
...s/rpc_sender/take_and_compare_snapshot_response.rs	0.00%	20 Missing ⚠️
rust/decentraland-godot-lib/src/dcl/js/mod.rs	0.00%	19 Missing ⚠️
...lib/src/godot_classes/rpc_sender/dcl_rpc_sender.rs	9.52%	19 Missing ⚠️
...st/decentraland-godot-lib/src/godot_classes/mod.rs	0.00%	11 Missing ⚠️
...raland-godot-lib/src/scene_runner/rpc_calls/mod.rs	0.00%	9 Missing ⚠️
rust/decentraland-godot-lib/src/dcl/mod.rs	0.00%	3 Missing ⚠️
...t/decentraland-godot-lib/src/scene_runner/scene.rs	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   13.03%   12.82%   -0.22%     
==========================================
  Files         111      116       +5     
  Lines       13874    14149     +275     
==========================================
+ Hits         1808     1814       +6     
- Misses      12066    12335     +269     

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[robtfm]

looks good, although the path being user:// is unix-centric.

if we had user agent in engine info (or some other way to determine the client) we could have the screenshots as scene content.

[kuruk-mm]

Looks great, minor reviews