test: add is_ipc_compiled() and skip_if_no_ipc() functions

Expose ENABLE_IPC build option to functional tests so new tests can test
IPC-only features.

test/config.ini.in

@@ -26,3 +26,4 @@ RPCAUTH=@abs_top_srcdir@/share/rpcauth/rpcauth.py
 @ENABLE_ZMQ_TRUE@ENABLE_ZMQ=true
 @ENABLE_EXTERNAL_SIGNER_TRUE@ENABLE_EXTERNAL_SIGNER=true
 @ENABLE_USDT_TRACEPOINTS_TRUE@ENABLE_USDT_TRACEPOINTS=true
+@ENABLE_IPC_TRUE@ENABLE_IPC=true