Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/wallet/test/wallet_test_fixture.h

@@ -10,7 +10,7 @@
 /** Testing setup and teardown for wallet.
  */
 struct WalletTestingSetup: public TestingSetup {
-    WalletTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
+    explicit WalletTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
     ~WalletTestingSetup();
 };