fix(unit_tests): point config tests at internal_test_data on this branch

[fruch]

Found while triaging the 40 integration failures on #15691 — this accounts for 30 of them, and is entirely independent of that PR.

Problem

Every test in unit_tests/integration/test_config_get_version_based_on_conf.py fails before reaching a single assertion:

FAILED unit_tests/integration/test_config_get_version_based_on_conf.py::test_docker[latest]
  - FileNotFoundError: Couldn't find config file:
    /home/ubuntu/scylla-cluster-tests/unit_tests/test_configs/minimal_test_case.yaml

unit_tests/test_configs/ does not exist on this branch. Master moved the fixtures with c16e41a55c "improvement(treewide): Move internal_test_data into unit_tests", which was never backported here — the yamls still live in internal_test_data/. The test file arrived by cherry-pick carrying master's path, so it could never have passed on this branch.

The two files are byte-identical (diff of branch-2025.1:internal_test_data/minimal_test_case.yaml against master:unit_tests/test_configs/minimal_test_case.yaml is empty), so the path is the only thing wrong.

Verification

Run inside the branch's own hydra image, scylladb/hydra:v1.91-2025.1-pytest-xdist:

	before	after
Couldn't find config file occurrences	33	0
result	33 failed, 0 passed	30 failed, 3 passed

The config now loads and the tests proceed into real validation. I am deliberately not claiming this turns 30 tests green: the remaining failures are live-lookup rot of the same class master fixed in #15670/#15671 — Azure Image for scylla_version='master:latest' not found in eastus, GCE image for scylla_version='2026.2' was not found, KeyError: 'docker-image-name' — plus, in my local run only, 15 failures from absent AWS credentials that CI does have. What this PR fixes is the blanket blocker that stopped every one of them from even getting that far.

Also in this change

• unit_tests/test_utils__operator__multitenant_common.py carried the same stale path. It is latent: setup_default_env sets it, but _multitenant_class_with_shared_options overwrites SCT_CONFIG_FILES before anything loads it. Confirmed neutral — that file gives an identical 4 failed / same reasons (Unable to locate credentials, local-only) with and without the change.
• AGENTS.md layout description updated to name internal_test_data/ instead of master's unit_tests/test_configs/.

Deliberately untouched

unit_tests/test_config_get_version_based_on_conf.py — the stale pre-rename duplicate that #15691 deletes. Fixing the path there too would create a modify/delete conflict between the two PRs.