Investigate expected TestGribMessage failures

[bjlittle]

Investigate the following iris.tests.integration.test_pickle.TestGribMessage expected failures:

• test_protocol_0
• test_protocol_1
• test_protocol_2

Reference PR #2608

[github-actions]

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[trexfeathers]

@pp-mo / @lbdreyer can this be closed? I believe you recently did some super work removing all GRIB stuff from core Iris?

[lbdreyer]

@trexfeathers yes this can be closed. This testing has been ported to iris-grib. There are some outstanding issues, but those are being tracked on iris-grib instead