Test refactor: separate Task validation tests for propagation

To support propagated parameters in inline Task specs, validation of variable usage
is skipped for inline Task specs and performed only for referenced Tasks.
This commit refactors TestTaskSpecValidateError to split out the test cases which rely
on this validation logic from test cases that rely on validation logic that is always performed.
Since validation logic for usage of declared variables should always be performed when a Task
is created directly (but not always for a Task spec), it updates these tests to validate a Task
rather than a Task spec. This refactoring will make cleanup for propagated parameters easier.
No functional changes.