Replace the usage of Common Test Strings with Named Constants

In the file eventing/tests/functional_tests/bucket_ops_test.go, the string dst_bucket is used very frequently. Hence, it would be beneficial to convert this string to a named constant defined in the file eventing/tests/functional_tests/defs.go like how we define and use the constant dstBucket.

Note:

• This ticket has been created as per the discussion in the Gerrit Patch discussion thread: https://review.couchbase.org/c/eventing/+/204416/comment/7f3f71dc_ebc47674/