HDDS-11702. Merge test_bucket_encryption into robot compatibility test

[adoroszlai]

What changes were proposed in this pull request?

test_bucket_encryption was added to xcompat/test.sh as separate step, but it can be performed as part of the existing write/read test.

Benefits:

• fewer top-level elements in Robot test report
• increase coverage by testing read with all versions

Also, remove duplicated code related to _init.

https://issues.apache.org/jira/browse/HDDS-11702

How was this patch tested?

xcompat-cluster-2.0.0-client-1.1.0-write :: Write Compatibility               
==============================================================================
Create Encrypted Bucket                                               | PASS |
------------------------------------------------------------------------------
Create Key in Encrypted Bucket                                        | PASS |
...
xcompat-cluster-2.0.0-client-2.0.0-read-1.1.0 :: Read Compatibility           
==============================================================================
Key Can Be Read                                                       | PASS |
------------------------------------------------------------------------------
Encrypted Key Can Be Read                                             | PASS |
...

CI:
https://github.com/adoroszlai/ozone/actions/runs/11882211273