Merge #18504: build: Drop bitcoin-tx and bitcoin-wallet dependencies on libevent

01a3392b1b Drop bitcoin-wallet dependency on libevent (Russell Yanofsky)
0660119ac3 Drop unintended bitcoin-tx dependency on libevent (Russell Yanofsky)

Pull request description:

  This fixes compile errors trying to build bitcoin-tx and bitcoin-wallet without libevent, which were reported by Luke Dashjr in https://github.com/bitcoin/bitcoin/issues/18465

  The fix avoiding `bitcoin-tx` dependency on libevent just adds a conditional build rule. This is implemented in the first commit (more details in commit description).

  The fix avoiding `bitcoin-wallet` dependency on libevent requires minor code changes, because `bitcoin-wallet` (unlike `bitcoin-tx`) links against code that calls `urlDecode` / `evhttp_uridecode`. This fix is implemented in the second commit (again details in the commit description).

ACKs for top commit:
  jonasschnelli:
    utACK 01a3392b1b.

Tree-SHA512: d2245e912ab494cccceeb427a1eca8e55b01a0006ff93eebcfb5461ae7cecd1083ac2de443d9db036b18bdc6f0fb615546caaa20c585046f66d234937f74870a

src/test/util/setup_common.cpp

@@ -27,12 +27,14 @@
 #include <util/string.h>
 #include <util/time.h>
 #include <util/translation.h>
+#include <util/url.h>
 #include <validation.h>
 #include <validationinterface.h>
 
 #include <functional>
 
 const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
+UrlDecodeFn* const URL_DECODE = nullptr;
 
 FastRandomContext g_insecure_rand_ctx;
 /** Random context to get unique temp data dirs. Separate from g_insecure_rand_ctx, which can be seeded from a const env var */