Unit test fixes for AssertLockHeld / -DDEBUG_LOCKORDER

Unit tests would fail if compiled with -DDEBUG_LOCKORDER (AssertLockHeld()
would fail; AssertLockHeld() relies on the DEBUG_LOCKORDER code to keep
track of locks held).

Fixed by LOCK'ing the wallet mutex in the unit tests that manipulate the
wallet.

src/test/wallet_tests.cpp

@@ -62,6 +62,8 @@ BOOST_AUTO_TEST_CASE(coin_selection_tests)
     CoinSet setCoinsRet, setCoinsRet2;
     int64_t nValueRet;
 
+    LOCK(wallet.cs_wallet);
+
     // test multiple times to allow for differences in the shuffle order
     for (int i = 0; i < RUN_TESTS; i++)
     {