Fix test_remove_pyramids_check_thumbs NFS

[pwalczysko]

What this PR does

Fixes https://merge-ci.openmicroscopy.org/jenkins/job/OMERO-test-integration/lastCompletedBuild/testReport/OmeroPy.test.integration.clitest.test_pyramids/TestRemovePyramidsFullAdmin/test_remove_pyramids_check_thumbnails/

Adds the waits _for_pyramid_file in 2 places and waits for thumbs deletion: necessary for NFS. The test was suffering from race problems in 3 places. The pyramids have to be

• generated after import
• deleted in order to comply with test setup (this will delete the created thumb too)
• regenerated (and with them the new thumb which is identical to the originally imported)

All 3 actions ^^ are slower on NFS and the necessary checks were added here so that the test can go ahead smoothly.

Testing this PR

Check that Jenkins is green.
Note: This was tested as passing in the Jenkins env itself (but the test was edited and triggered manually).

Related reading

Similar to #6464, but a bit more complex, as this ...check_thumbnails test is multi-staged.

[pwalczysko]

Adding Exclude label, as this might conflict with #6471

[pwalczysko]

closing in favor of #6471 - the strategy taken in #6471 is working and is superior to the long wait loop introduced here.