rearranged unit-agrad-* tests

#### Summary:

Moved src/test/unit-agrad-rev/* to src/test/unit/agrad/rev.
Moved src/test/unit-agrad-fwd/* to src/test/unit/agrad/fwd.

#### Intended Effect:

The old layout made it hard to remember where the agrad tests were.

#### How to Verify:

Run the tests by typing:
```
./runTests.py src/test/unit/agrad
```

#### Side Effects:

This will affect Jenkins, but I can handle that.
This may affect existing pull requests, but I can also handle that.

#### Documentation:

Not user facing.

#### Reviewer Suggestions: 

Bob.