[regression-test](backup-restore) wait for colocate group to stabilize before asserting COLOCATE plan

[shuke987]

Problem

test_backup_restore_colocate_with_partition (in regression-test/suites/backup_restore/test_backup_restore_colocate.groovy) flakily fails right after a RESTORE:

Explain and check failed, expect contains 'COLOCATE', but actual explain string is:
  HAS_COLO_PLAN_NODE: false
  3:VHASH JOIN(331)
  |  join op: INNER JOIN(BROADCAST)[]
  TABLE: ..._db_new..._table1

(reproduced deterministically in an isolated run on a 4-BE / force-3 cluster; failure was on the restore-to-new-db case.)

Root cause — a case timing bug, not a plan regression

After RESTORE the restored colocate group needs a moment to become stable, and the planner only emits a COLOCATE join once the group is stable (otherwise it falls back to BROADCAST/shuffle). The suite ran explain ... contains("COLOCATE") immediately after waitAllRestoreFinish, racing the stabilization. The existing checkColocateTabletHealth (a single-shot ColocateMismatchNum == 0 assert) sat after the assertion, so it didn't gate the explain.

Fix

• Add a bounded poll waitColocatePlan(query) (60 × 1s) that waits for the explain plan to actually contain COLOCATE, and call it before each contains("COLOCATE") assertion.
• Turn checkColocateTabletHealth into a bounded poll as well, so the health check waits for stabilization instead of racing it.
• Applied symmetrically to both suites in the file; the notContains("COLOCATE") assertions are untouched.

If COLOCATE never appears within the timeout, the assertion still fails — so a genuine regression would not be masked.

Verification

Run isolated on the cluster where the original case failed deterministically: the fixed case passes (Test 1 suites, failed 0 suites).

[hello-stephen]

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[shuke987]

run buildall