Add parallel garbage collection test runner

[JoshuaBatty]

Description

Adds a comprehensive test suite that exercises garbage collection across all language test examples. The test:

• Runs each example in its own isolated process to prevent cross-contamination
• Uses rayon for parallel execution, reducing total test time to ~60s for all 267 tests
• Provides a clear summary of passing/failing tests with specific error cases
• Continues running all tests even when individual tests fail

The test identified 26 examples that need investigation for proper garbage collection handling, particularly around:

• Constants and their declarations
• Module dependencies
• Type system (arrays, slices, strings)
• Associated constants
• References and self implementations

By running tests in isolation, we can now identify all problematic cases rather than stopping at the first failure.

The current projects that have garbage collection failures are:

associated_const_trait_const
references
reexport
main_args
fixing_generic_type
shadowing
far_jumps
str_slice
match_expressions
slice
const_decl_literal
raw_ptr
import_star_name_clash
module_dep_self
module_dep_multiple
self_impl_reassignment
const_decl
associated_const_abi_default
string_slice
fix_opcode_bug
tuple_field_reassignment
const_decl_with_call_path
module_dep
array_basics
u256
associated_const_impl_self_order

Note: I've temporarily disabled the run_all_garbage_collection_tests so this PR can be reviewed and pass CI. Then #6613 can rebase and reenable the test to ensure that PR allows these failing tests to pass.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6665 will not alter performance

_{Comparing josh/comprehensive_gc_tests (d739eee) with master (058f4e2)}

Summary

✅ 22 untouched benchmarks

[sdankel]

Nice work! It'll be good to have tests ensure these type of GC issues are solved once and for all.

[ironcev]

I love the simplicity and the usefulness of this solution!