Introduce MockableDatabase for wallet unit tests

MockableDatabase is a WalletDatabase that allows us to interact with the
records to change them independently from the wallet, as well as
changing the return values from within the tests. This will give us
greater flexibility in testing the wallet.

build_msvc/libtest_util/libtest_util.vcxproj.in

@@ -8,6 +8,7 @@
     <ConfigurationType>StaticLibrary</ConfigurationType>
   </PropertyGroup>
   <ItemGroup>
+    <ClCompile Include="..\..\src\wallet\test\util.cpp" />
 @SOURCE_FILES@
   </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />