Added Bridge Validators deterministic mock for e2e tests

contracts/mocks/BridgeValidatorsDeterministic.sol

@@ -0,0 +1,13 @@
+pragma solidity 0.4.24;
+
+import "../upgradeable_contracts/BridgeValidators.sol";
+
+contract BridgeValidatorsDeterministic is BridgeValidators {
+    function isValidatorDuty(address _validator) external view returns (bool) {
+        address next = getNextValidator(F_ADDR);
+        require(next != address(0));
+
+        // first validator is always on duty, others are always not
+        return _validator == next;
+    }
+}