Fix ndf in TestDefragInterrupted

BUG FIX

Description of Bug and Fix

TestDefragInterrupted was failing for:

--- FAIL: TestDefragInterrupted (11.72s)
    defrag_test.go:299: defrag should result in fewer than defragThreshold outputs, got 53 wanted 50

The failure was that the values in the build.Retry were never changing. The fix was to remove the sleep and move the block mined into the build.Retry

Testing Added

N/A

Proof of Fix (ie Screenshot if applicable)

https://gitlab.com/NebulousLabs/Sia/-/jobs/114570675

Changes to Work Flow (if applicable)

N/A