add unit tests for WriteBatch

[pfi79]

hyperledger/fabric#5086

[pfi79]

@bestbeforetoday Take a look. What other unit tests are missing.

[bestbeforetoday]

Generally coverage looks good. A huge improvement on what was there before. The only case I noticed missing is that starting a write batch twice doesn't lose any previously batched writes.

An area that could (optionally) be improved is how specific the tests are. Right now, each unit tests tends to perform a significant number of operations. This means that failures are quite generic and don't pinpoint specific problems in the code well, meaning that debugging of the code is probably going to be needed to identify the actual problem and fix any test failure. The smaller and more targeted each unit test is, the more accurately the detected issue can be identified and more quickly it can be fixed. The ideal would be that a test failure message would be enough information to know exactly the conditions that failed and therefore be able to identify the faulty code without any debugging. Perhaps that's just something to consider for future tests though.

[pfi79]

The only case I noticed missing is that starting a write batch twice doesn't lose any previously batched writes.

added

[pfi79]

@denyeart look at this pr. I think we can put a tag after it. And we can move on to fabric changes.