DAOS-18304 ddb: add C unit tests for ddb_run_open

[knard38]

Description

This PR builds on top of #18124.
While working on that refactoring, it became apparent that ddb_run_open lacked C-level unit test coverage.
This patch addresses that gap.

It adds open_cmd_tests to ddb_commands_tests.c, covering the four main behaviors of ddb_run_open:

• Invalid path: opening a non-existent VOS file returns DER_INVAL.
• Read-only open: a successful open with write_mode=false leaves dc_poh valid and dc_write_mode set to false in the context.
• Write-mode open: a successful open with write_mode=true leaves dc_poh valid and dc_write_mode set to true in the context.
• Pool already open: attempting to open a pool when one is already open returns DER_BUSY.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'Add unit test to ddb go code'
Status is 'In Review'
https://daosio.atlassian.net/browse/DAOS-18304