Tests/Instrumentation: Fix exponential output spam from validate_json_schema

Schema validations in Tests/Instrumentation called add_error("${RunCMake_TEST_FAILED}")
in a loop. Because add_error itself appends the input to RunCMake_TEST_FAILED, this
resulted in exponential growth of error messages on each loop iteration, filling the
test log with duplicate errors.

Isolate error messages from validate_json_schema to avoid exponential duplication.