Move client integration test declarations out of the large harness files into
focused files for cache, exporters, sources, mounts, gateway behavior, and
related client features.
Keep registration lists grouped by source file, move non-gateway tests out of
the gateway harness, and remove the now-empty build_test.go. This is intended
as a mechanical split with no test logic changes.
Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
