Add "test_delete_agent_negative"

In [0], "AgentManagementTestJSON" was removed from tempest to avoid
duplications with this repository, but "test_delete_agent_negative"
was missing.

[0] https://review.opendev.org/#/c/698589/

Change-Id: I2a6e0f958dac8fb41bdd709e7b595e256b84ed03
Related-Bug: #1855985
1 file changed