test: move and simplify BOOST_CHECK ostream helpers

Move the operator<< overloads used by BOOST_CHECK_* out of the
unit test machinery test/setup_common, into test/util/common.h.

And replace the individual per-type ToString() overloads with
a single concept-constrained template that covers any type
exposing a ToString() method. This is important to not add
uint256.h and transaction_identifier.h dependencies to the
shared test/util/common.h file.

Co-authored-by: furszy <matiasfurszyfer@protonmail.com>

src/test/rpc_tests.cpp

@@ -9,6 +9,7 @@
 #include <rpc/client.h>
 #include <rpc/server.h>
 #include <rpc/util.h>
+#include <test/util/common.h>
 #include <test/util/setup_common.h>
 #include <univalue.h>
 #include <util/time.h>