Add test/README.md (TS-07): file naming, test/support/ and test/fixtures/, the notifier-subclass override pattern, what the source-text contract tests pin, FFmpeg, python3 and SMB requirements, running a subset, and the timing of library_browse_sqlite_paging_test.
Derived from test/support/*.dart, test/fixtures/, test/source_contracts_test.dart, test/design_system_test.dart, the FFmpeg and SMB suites, test/library_browse_sqlite_paging_test.dart (timed at 25 s locally) and .github/workflows/ci.yml.
