fix(ci): fix failing gateway integration test (#4358)

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Author: tomg10

core/tests/highlevel-test-tools/tests/gateway-migration-test.test.ts

@@ -23,6 +23,8 @@ if (shouldSkip) {
 
         await testChain.waitForAllBatchesToBeExecuted();
 
+        await testChain.mainNode.kill();
+
         await gatewayMigrationFromGatewayTest(testChain.chainName);
 
         await gatewayMigrationToGatewayTest(testChain.chainName);