adds test to cover deleting unsplittable col in split code

[keith-turner]

The new test in this commit covers the following code

accumulo/server/manager/src/main/java/org/apache/accumulo/manager/tableOps/split/FindSplits.java

Lines 142 to 148 in d1456fc

	log.info("Tablet {} no longer needs to split, deleting unsplittable marker.",
	tabletMetadata.getExtent());
	var mutator = tabletsMutator.mutateTablet(extent).requireAbsentOperation()
	.requireSame(tabletMetadata, FILES).deleteUnSplittable();
	mutator.submit(tm -> tm.getUnSplittable() == null);
	// Case 3: The table config and/or set of files changed since the tablet mgmt iterator
	// examined this tablet.

[keith-turner]

As a result of running this test and looking at the coverage data #4792 and #4793 were opened. The test will run without those changes, but will not get full code coverage.

[cshannon]

This is a nice test to add to confirm deletion works when the threshold changes to go along with the test that the unsplittable column was added when the tablet can't be split. I went back and took a look and there is an existing IT that covers this case but I forgot to add one to the ManagerRepoIT after TestAmple was created.

So now we have tests for the column creation/deletion both using TestAmple to simulate things as well as with an IT that uses the real system Ample and metadata which is nice.

[keith-turner]

I went back and took a look and there is an existing IT that covers this case but I forgot to add one to the ManagerRepoIT after TestAmple was created.

That was a good find. I added links in the javadoc for each test pointing to the other. Since the test are somewhat similar it may be useful to know about the other test when making changes in the future.