Large predicate move test

A long-running integration test for the size-aware predicate move timeout and rebalancer backoff (dgraph-io/dgraph#9792, fixes #9784). It is excluded from CI via the largemove build tag, which no workflow compiles: the test loads several GiB of data and can run from tens of minutes to hours depending on the data size and host.

What it does

1. Brings up a 1-zero, 2-alpha cluster (one group per alpha) via dgraphtest and loads MOVE_TEST_GB GiB (default 8) of incompressible 64KiB string values into one predicate.
2. Waits until Zero reports the tablet size, then triggers a move and kills the destination Alpha mid-stream. Asserts the move fails and Zero records the rebalancer backoff ("Skipping automatic rebalancing of this tablet").
3. Restarts the destination and retries. Asserts the move completes, every move announcement in Zero's log carried a size-scaled timeout above the 2h floor, and the row count is intact on the destination group.

The auto-rebalancer cannot interfere: with a single dominant tablet, chooseTablet only picks tablets no larger than half the group size difference, which the payload tablet always exceeds.

Running

make install       # builds the binary dgraphtest mounts into containers
make image-local   # dgraphtest boots containers from dgraph/dgraph:local; build it if absent
go test -v -timeout=8h --tags=largemove -run TestLargePredicateMove ./systest/predicate-move/

Do not run this through make test TAGS=largemove: the Makefile does not pass a -timeout, so the Go default of 10m kills the test.

Knobs and requirements:

• MOVE_TEST_GB (default 8, minimum 3): value data loaded before the move. This is a floor, not the final size: the test measures the host's ingest rate and tops up automatically until the estimated move duration outlasts the 75s kill point (roughly 17 GiB on a fast laptop NVMe, less on slower disks).
• Disk: budget roughly 5x the final loaded size inside the Docker Desktop VM (source + destination copies, Raft WAL, and compaction headroom); on a fast host assume ~80-100 GB.
• Time: about 30 minutes at the default size on an Apple Silicon laptop, dominated by the load and the tablet-size reporting interval (Alphas recompute tablet sizes on a periodic ticker).