test: Pass bench exe into test framework utils

This teaches the test framework about the bench executable, which is
required for the next commit.

test/config.ini.in

@@ -16,6 +16,7 @@ RPCAUTH=@abs_top_srcdir@/share/rpcauth/rpcauth.py
 [components]
 # Which components are enabled. These are commented out by cmake if they were disabled during configuration.
 @ENABLE_WALLET_TRUE@ENABLE_WALLET=true
+@BUILD_BENCH_TRUE@BUILD_BENCH=true
 @BUILD_BITCOIN_CLI_TRUE@ENABLE_CLI=true
 @BUILD_BITCOIN_TX_TRUE@BUILD_BITCOIN_TX=true
 @BUILD_BITCOIN_UTIL_TRUE@ENABLE_BITCOIN_UTIL=true