Scope ExUnit diff width tests

[evadne]

Summary

• Make ExUnit.DiffTest use :infinity as the default diff formatting width.
• Add an explicit terminal-width override for tests that exercise display layout.
• Flatten generated-value expectations for refs and functions so VM-generated inspect strings do not control wrapping.

Context

This follows up on #13954 which changed the test helper to format at 80 columns globally to force-test the inserted/deleted map-in-list formatting. However, this introduced a latent issue in diffs that involve PIDs or Refs that have variable lengths, and such tests may fail intermittently such as in https://github.com/elixir-lang/elixir/actions/runs/25930787536/job/76225550506:

1) test refs (ExUnit.DiffTest)
   test/ex_unit/diff_test.exs:1221
   Assertion with =~ failed
   code:  assert diff_left =~ expected_left
   left:  "{-#Reference<0.134765775.2.103458>-, -#Reference<0.134765775.2.103459>-}"
   right: "{\n  -#Reference<0.134765775.2.103458>-,\n  -#Reference<0.134765775.2.103459>-\n}"
   stacktrace:
     test/ex_unit/diff_test.exs:123: ExUnit.DiffTest.refute_diff/5
     test/ex_unit/diff_test.exs:1233: (test)

This PR keeps that finite-width coverage, but narrows it to deterministic layout assertions:

• structs with missing keys on match
• maps in lists
• structs in lists

Other diff tests default back to :infinity, which makes semantic checks independent from generated inspected values such as refs, pids, and functions.

Tests

• make test_ex_unit
• bin/mix format --check-formatted lib/ex_unit/test/ex_unit/diff_test.exs
• git diff --check

[josevalim]

💚 💙 💜 💛 ❤️