experiment: two additional tests for graph-copy

[crusso]

Two additional tests for @luc-blaeser's graph copying branch (luc/graph-copy), Builds on #4286.

• run-drun/stable-exp-immut.mo is a version of stable-exp that uses immutable arrays, to highlight the sharing of those arrays that would previously explode stabilization (using Candidish) (as suggested in a comment in stable-exp.mp)
• run-drun-non-ci/stable-var-large.mo attempts to allocated more than 2 GB of stable variables and then upgrade. This should succeed with graph-copy but fail with Candidish due to non-streaming destabilization (which requires copying all stable variable data to the heap before deserialization). Unfortunately, I can't execute it on my machine without causing drun to choke (possible due a lack of diskspace). Disabled for now by placing in attic run-drun-non-ci.

[github-actions[bot]]

Comparing from b2a8b242ee1bba6ca337f73169ddb58c2c63fb05 to ba4c4c7084466333c1eedd338410fe8573d722de: The produced WebAssembly code seems to be completely unchanged.