Use higher priority for PVF preparation in dispute/approval context (#4172)

Related to https://github.com/paritytech/polkadot-sdk/issues/4126
discussion

Currently all preparations have same priority and this is not ideal in
all cases. This change should improve the finality time in the context
of on-demand parachains and when `ExecutorParams` are updated on-chain
and a rebuild of all artifacts is required. The desired effect is to
speed up approval and dispute PVF executions which require preparation
and delay backing executions which require preparation.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/core/candidate-validation/src/tests.rs

@@ -368,6 +368,7 @@ impl ValidationBackend for MockValidateCandidateBackend {
 		_pvf: PvfPrepData,
 		_timeout: Duration,
 		_encoded_params: Vec<u8>,
+		_prepare_priority: polkadot_node_core_pvf::Priority,
 	) -> Result<WasmValidationResult, ValidationError> {
 		// This is expected to panic if called more times than expected, indicating an error in the
 		// test.
@@ -1044,6 +1045,7 @@ impl ValidationBackend for MockPreCheckBackend {
 		_pvf: PvfPrepData,
 		_timeout: Duration,
 		_encoded_params: Vec<u8>,
+		_prepare_priority: polkadot_node_core_pvf::Priority,
 	) -> Result<WasmValidationResult, ValidationError> {
 		unreachable!()
 	}