Fix ddev testable command to properly use the tag, fallback on the branch if absent.

[remeh]

What does this PR do?

Fix ddev testable command to properly use the tag (and fallback on the branch if absent).

Motivation

It could use the branch with the same name instead.

Additional Notes

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
  - [ ] If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #4775 into master will decrease coverage by 17.07%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
vertica/tests/metrics.py
riakcs/datadog_checks/riakcs/riakcs.py
oracle/tests/test_oracle.py
twistlock/datadog_checks/twistlock/twistlock.py
linkerd/datadog_checks/linkerd/linkerd.py
istio/tests/test_e2e.py
gitlab_runner/tests/test_integration.py
...ent_log/datadog_checks/win32_event_log/__init__.py
btrfs/tests/test_btrfs.py
...nmetrics/datadog_checks/openmetrics/openmetrics.py
... and 805 more