tests: fs mock: Raise an assertion when a folder/file can't be found

This matches the unix fs actual implementation

test/mocks/FileSystem.h

@@ -165,7 +165,7 @@ struct FileSystemFactory : public fs::IFileSystemFactory
     {
         auto d = device( mrl );
         if ( d == nullptr )
-            return std::make_shared<Directory>( "", nullptr );
+            throw std::system_error{ ENOENT, std::generic_category(), "Mock directory" };
         auto dir = d->directory( mrl );
         if ( dir == nullptr )
             throw std::system_error{ ENOENT, std::generic_category(), "Mock directory" };

test/mocks/filesystem/MockDevice.cpp

@@ -25,6 +25,7 @@
 #endif
 
 #include <cassert>
+#include <system_error>
 
 #include "MockDevice.h"
 #include "MockDirectory.h"
@@ -106,7 +107,7 @@ std::shared_ptr<File> Device::file(const std::string& filePath )
 std::shared_ptr<Directory> Device::directory(const std::string& path)
 {
     if ( m_root == nullptr || m_present == false )
-        return std::make_shared<Directory>( "", nullptr );
+        throw std::system_error{ ENOENT, std::generic_category(), "Mock directory" };
     const auto relPath = relativePath( path );
     if ( relPath.empty() == true )
         return m_root;

test/mocks/filesystem/MockDirectory.cpp

@@ -152,7 +152,7 @@ std::shared_ptr<Directory> Directory::directory(const std::string& path)
     {
         auto it = m_dirs.find( subFolder );
         if ( it == end( m_dirs ) )
-            return std::make_shared<Directory>( "", nullptr );
+            throw std::system_error{ ENOENT, std::generic_category(), "Mock directory" };
         return it->second;
     }
     else