Reinitialize state in between individual unit tests.

This changes the TestingSetup fixture to be per-unit-test rather than global.
Most tests don't need it, so it's only invoked in a few.

src/test/test_bitcoin.h

@@ -0,0 +1,18 @@
+#ifndef BITCOIN_TEST_TEST_BITCOIN_H
+#define BITCOIN_TEST_TEST_BITCOIN_H
+
+#include "txdb.h"
+
+#include <boost/filesystem.hpp>
+#include <boost/thread.hpp>
+
+struct TestingSetup {
+    CCoinsViewDB *pcoinsdbview;
+    boost::filesystem::path pathTemp;
+    boost::thread_group threadGroup;
+
+    TestingSetup();
+    ~TestingSetup();
+};
+
+#endif