Add more split integration tests

[keith-turner]

The following TODOs about adding more test were added by #3382

accumulo/server/manager/src/main/java/org/apache/accumulo/manager/split/SplitUtils.java

Line 191 in 808c0bb

// ELASTICITY_TODO tablets used to always split into 2 tablets. Now the split operation will

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

Line 68 in 808c0bb

// ELASTICITY_TODO write IT that spins up 100 threads that all try to add a diff split to

[DomGarguilo]

I've started working on adding tests for these cases in #3415

@keith-turner can you please take a look to see if I am on the right track to address the second case mentioned above?

[DomGarguilo]

This ticket has been partially addressed by #3415. The TODO in SplitUtils is still unaddressed:

accumulo/server/manager/src/main/java/org/apache/accumulo/manager/split/SplitUtils.java

Line 191 in 808c0bb

// ELASTICITY_TODO tablets used to always split into 2 tablets. Now the split operation will