Fix test cleanup to prevent conflicts between tests.

[vbabanin]

JAVA-5761

[jyemin]

Approved, but wondering whether ClientSideEncryptionExternalKeyVaultTest should also do some cleanup, e.g. drop the collection.

[jyemin]

Let me know why the collection name change had to be reverted.

[vbabanin]

Let me know why the collection name change had to be reverted.

Initially, the calculation of bytes to be subtracted from the available bytes for the message in the batch-splitting test was defined as 1122 in the spec tests. After changing the namespace, the test started failing. I reverted the change but later found a way to recalculate it. The test and namespace have now been updated accordingly.