test: Add test for IPC serialization bug

Add regression test for serialization bug in IPC mining code that is not
currently being called anywhere reported:

Sjors/bitcoin#71
bitcoin-core/libmultiprocess#122

Author: ryanofsky

src/test/ipc_test.capnp

@@ -20,4 +20,5 @@ interface FooInterface $Proxy.wrap("FooImplementation") {
     passTransaction @3 (arg :Data) -> (result :Data);
     passVectorChar @4 (arg :Data) -> (result :Data);
     passBlockState @5 (arg :Mining.BlockValidationState) -> (result :Mining.BlockValidationState);
+    passScript @6 (arg :Data) -> (result :Data);
 }

src/test/ipc_test.cpp

@@ -121,6 +121,10 @@ void IpcPipeTest()
     BOOST_CHECK_EQUAL(bs3.GetRejectReason(), bs4.GetRejectReason());
     BOOST_CHECK_EQUAL(bs3.GetDebugMessage(), bs4.GetDebugMessage());
 
+    auto script1{CScript() << OP_11};
+    auto script2{foo->passScript(script1)};
+    BOOST_CHECK_EQUAL(HexStr(script1), HexStr(script2));
+
     // Test cleanup: disconnect pipe and join thread
     disconnect_client();
     thread.join();

src/test/ipc_test.h

@@ -6,6 +6,7 @@
 #define BITCOIN_TEST_IPC_TEST_H
 
 #include <primitives/transaction.h>
+#include <script/script.h>
 #include <univalue.h>
 #include <util/fs.h>
 #include <validation.h>
@@ -19,6 +20,7 @@ class FooImplementation
     CTransactionRef passTransaction(CTransactionRef t) { return t; }
     std::vector<char> passVectorChar(std::vector<char> v) { return v; }
     BlockValidationState passBlockState(BlockValidationState s) { return s; }
+    CScript passScript(CScript s) { return s; }
 };
 
 void IpcPipeTest();