Fix flaky migration test

[chrisdavies]

This PR fixes this flaky test:

fail: "apis saved_objects Kibana index migration Coordinates migrations across the Kibana cluster"
19:28:52              │ 
19:28:52              │       AssertionError: expected [ Array(2) ] to deeply equal [ Array(2) ]
19:28:52              │       + expected - actual
19:28:52              │ 
19:28:52              │        [
19:28:52              │          {
19:28:52              │       +    "destIndex": ".migration-c_2"
19:28:52              │       +    "status": "migrated"
19:28:52              │       +  }
19:28:52              │       +  {
19:28:52              │            "destIndex": [undefined]
19:28:52              │            "status": "skipped"
19:28:52              │          }
19:28:52              │       -  {
19:28:52              │       -    "destIndex": ".migration-c_2"
19:28:52              │       -    "status": "migrated"
19:28:52              │       -  }
19:28:52              │        ]
19:28:52              │       
19:28:52              │       at Function.assert.deepEqual (node_modules/chai/lib/chai/interface/assert.js:216:32)
19:28:52              │       at Context.it (test/api_integration/apis/saved_objects/migrations.js:183:14)
19:28:52              │       at <anonymous>
19:28:52              │       at process._tickCallback (internal/process/next_tick.js:188:7)
19:28:52              │ 

[chrisdavies]

retest

[chrisdavies]

Er. Looks like this was fixed by Spencer: #23347

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request