This commit replaces the REST test that the global checkpoint sync
action runs successfully as a privileged user. The test needs to be
replaced because it has a small race condition. Namely, the check that
the post-operation global checkpoint sync was successful could run
before the sync finishes running. To address this, we replace the REST
test with a test where we have a little more control and can assert busy
to avoid this race from failing the test.
Relates elastic/x-pack-elasticsearch#2749
Original commit: elastic/x-pack-elasticsearch@ea585b843c
c35efb7adf