banman: pass the banfile path in

There's no need to hard-code the path here. Passing it in means that there are
no ordering concerns wrt establishing the datadir.

src/test/test_bitcoin.cpp

@@ -94,7 +94,7 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         for (int i=0; i < nScriptCheckThreads-1; i++)
             threadGroup.create_thread(&ThreadScriptCheck);
 
-        g_banman = MakeUnique<BanMan>(nullptr);
+        g_banman = MakeUnique<BanMan>(GetDataDir() / "banlist.dat", nullptr);
         g_connman = MakeUnique<CConnman>(0x1337, 0x1337); // Deterministic randomness for tests.
 }