This PR modifies every test in `builtin/credentials/approle/path_role_test.go` with new validation checks to ensure that approle/path_role successful responses align with the declared response schema.
It also introduces a test helper in `sdk/helper/testhelpers`:
```go
func FindResponseSchema(t *testing.T, ...)
```
This test helper will be useful for all plugins that require similar response schema validation in tests.
### Background
This PR is part of the ongoing work to add structured responses in Vault OpenAPI (VLT-234)
