fix(server): defrag unit test fix

[boazsade]

Signed-off-by: Boaz Sade [email protected]
fix issue #571

There was an issue with the defrag unit test that when run by its own it would not start the task that collect the use memory stats from all shards. This would result in the task for defrag not running because the use memory == 0.
On top of it, there was an issue that since this started with empty memory the defrag task was not ran by other tests, and so the counter for running the defrag test was not activated.
Note that since we're going to rewrite the logic for defrag, this is going to change soon again, but for now, it required that this test will work correctly.
To ensure that the test is ran with less memory the flag that control the memory threshold is set as well.
The test also set enough memory so that we would not have issue with setting all the keys.