Runs all test files sequentially, each in its own OS process.

This avoids SQLite's global VFS contention that causes spurious "out of memory" errors when test files run in parallel.

Usage

mix test.seq
mix test.seq --trace
mix test.seq --cover   # per-file .coverdata exports under cover/

With --cover, each file's OS process exports a distinctly named .coverdata (derived from the file path), so runs don't overwrite each other. Merge and publish afterwards, e.g. mix coveralls.github --import-cover cover test/xqlite_test.exs.