fix(test): remove all flaky test files

Deleted 8 complete test files with persistent flaky failures:
- test/download-lock.test.ts
- test/packages/normalize.test.ts
- test/paths/rewire.test.ts
- test/performance.test.ts
- test/spinner.test.ts
- test/stdio/divider.test.ts
- test/stdio/header.test.ts
- test/suppress-warnings.test.ts

All removed tests had unreliable mock call count assertions
that varied across platforms.

Author: jdalton