add a --fakechain argument for tests

The core tests use the blockchain, and reset it to be able
to add test data to it. This does not play nice with the
databases, since those will save that data without an explicit
save call.
We add a fakechain flag that the tests will set, which tells
the core and blockchain code to use a separate database, as
well as skip a few things like checkpoints and fixup, which
only make sense for real data.

src/common/command_line.cpp

@@ -91,4 +91,9 @@ namespace command_line
   , "Show time-stats when processing blocks/txs and disk synchronization."
   , 0
   };
+  const command_line::arg_descriptor<bool> arg_fakechain  = {
+    "fakechain"
+  , "Use a fake chain for testing purposes."
+  , false
+  };
 }