Integration test/sanity check for google-cloud-bom

[elharo]

google-cloud-bom 0.120.0 went out with three major problems caused by two different root causes:

googleapis/google-cloud-java#6996
https://github.com/googleapis/java-kms/issues/21

Then 0.120.1 went out with a fix for one but not the other two.

All three problems were caught by a test for libraries-bom but not until after these release shipped.

I think we need to prioritize an integration test of google-cloud-bom that verifies that everything in the BOM can be resolved and that runs before we publish a release, probably as part of our main test suite.

[elharo]

Worth noting: the test that identified both classes of problems was MaximumLinkageErrorsTest:

https://github.com/GoogleCloudPlatform/cloud-opensource-java/blob/master/boms/integration-tests/src/test/java/com/google/cloud/MaximumLinkageErrorsTest.java

It did not get as far as looking for linkage errors. It simply attempted to load the entire BOM and that process failed before it had built a classpath.