test: normalize the suite titles of the helm unit tests

The contributor guidelines never defined how a `suite` title has to look, only how an `it` title has to look.
Without a rule five competing schemes grew side by side: `config template | cache config`, `deployment
template (probes)`, `ServiceMonitor template (basic)`, `Test Gateway API httpRoute.yaml` and `Dependency checks
| Customization integrity | valkey`. 24 of the 49 titles started with a lowercase letter, which contradicts the
sentence rule that already applies to the test titles.

More than cosmetics, the missing rule produced three concrete defects:

- `deployment template` was used by both deployment-additional-config.yaml and extraInitContainers.yaml, and
  `Init template` by both init_directory_structure.sh.yaml and its rootless counterpart. `helm unittest`
  identifies a failing suite by its title, so a failure in either file printed the same header and the report
  did not say which file to open.
- deployment/inline-config.yaml was titled `config template`, although it lives in the deployment directory and
  asserts on secret_inlineConfig.yaml. It read like a sixth member of the `config template | ...` group.
- persistentVolumeClaim.yaml was titled `PVC template`, using exactly the short name the chart conventions
  forbid for files and keys.

All suites now follow `Test <subject> <scenario>`, for example `Test deployment probes` or `Test gateway API
httpRoute`. The subject is the template under test, the scenario is what distinguishes the suite from its
siblings, so every title is unique and sorts next to its related suites. The `Test ` prefix of the Gateway API,
ingress and route suites is kept, because it is the only part of the old schemes that already read as a
sentence; the redundant `.yaml` suffix and the `tests` suffix of the storage class suite are dropped.

The rule is written down in the Unit Tests section of the copilot instructions, so the next suite does not have
to guess which of the five schemes was meant.

Co-authored-by: Copilot <copilot@github.com>

unittests/helm/metric-secret/metrics-secret-servicemonitor-disabled.yaml

@@ -1,4 +1,4 @@
-suite: Metrics secret template (monitoring disabled)
+suite: Test metrics secret with monitoring disabled
 release:
   name: gitea-unittests
   namespace: testing

unittests/helm/metric-secret/metrics-secret-servicemonitor-enabled.yaml

@@ -1,4 +1,4 @@
-suite: Metrics secret template (monitoring enabled)
+suite: Test metrics secret with monitoring enabled
 release:
   name: gitea-unittests
   namespace: testing