test: fill coverage gaps in `languages/javascript`

[KRRT7]

Parent: #2079

Phase 3 — languages/javascript to 100%

Module	Stmts	Current coverage
languages/javascript	5,797	71%

Key files needing attention:

• javascript/comparator.py (85 stmts, 0%)
• javascript/edit_tests.py (309 stmts, 25%)
• javascript/optimizer.py (28 stmts, 25%)
• javascript/function_optimizer.py (106 stmts, 61%)
• javascript/tracer.py (83 stmts, 61%)
• javascript/parse.py (259 stmts, 65%)
• javascript/vitest_runner.py (325 stmts, 66%)
• javascript/support.py (1,156 stmts, 68%)
• javascript/test_runner.py (561 stmts, 68%)
• javascript/instrument.py (637 stmts, 71%)
• javascript/import_resolver.py (235 stmts, 72%)
• javascript/normalizer.py (104 stmts, 74%)
• javascript/line_profiler.py (99 stmts, 75%)
• javascript/module_system.py (213 stmts, 80%)

Target

100% coverage for languages/javascript (~5,797 statements).