test: Create test fixture for wallet

Removes all the `#ifdef ENABLE_WALLET` from `test_bitcoin` by
making the wallet tests use their own fixture.

src/test/test_bitcoin.h

@@ -25,8 +25,7 @@ struct BasicTestingSetup {
 };
 
 /** Testing setup that configures a complete environment.
- * Included are data directory, coins database, script check threads
- * and wallet (if enabled) setup.
+ * Included are data directory, coins database, script check threads setup.
  */
 struct TestingSetup: public BasicTestingSetup {
     CCoinsViewDB *pcoinsdbview;