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/util.h

@@ -94,6 +94,7 @@ bool RenameOver(boost::filesystem::path src, boost::filesystem::path dest);
 bool TryCreateDirectory(const boost::filesystem::path& p);
 boost::filesystem::path GetDefaultDataDir();
 const boost::filesystem::path &GetDataDir(bool fNetSpecific = true);
+void ClearDatadirCache();
 boost::filesystem::path GetConfigFile();
 #ifndef WIN32
 boost::filesystem::path GetPidFile();