test: style pass on fused-pipeline tests

Hoist the imports that nearly every test re-imported locally, add a
shared _make_spec helper to replace nine copies of the ArraySpec
boilerplate, and convert test_async_chunk_transform_matches_sync to
Expect cases where each case declares its expected byte-comparison
function (exact equality, or gzip-MTIME-tolerant), removing the
isinstance branch from the test body.

Also add the missing read-back assertion in
test_sync_write_async_read_roundtrip, which previously read into a
buffer and never compared it to the written data.

Assisted-by: ClaudeCode:claude-fable-5

Author: d-v-b