hwmonio: Add injection point to test internal behavior

With the complexities of the various behaviors within the read() method
of HwmonIO, introduce an injection point for testing.  There is a
default available, and therefore this is a surgical change that only
impacts future tests.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I1ead56c7fe1a2f87ebf316488e68f435a41c9d19
diff --git a/test/filesystem_mock.hpp b/test/filesystem_mock.hpp
new file mode 100644
index 0000000..9d24d3c
--- /dev/null
+++ b/test/filesystem_mock.hpp
@@ -0,0 +1,19 @@
+#pragma once
+
+#include "hwmonio.hpp"
+
+#include <string>
+
+#include <gmock/gmock.h>
+
+namespace hwmonio
+{
+
+class FileSystemMock : public FileSystemInterface
+{
+  public:
+    MOCK_CONST_METHOD1(read, int64_t(const std::string&));
+    MOCK_CONST_METHOD2(write, void(const std::string&, uint32_t));
+};
+
+} // namespace hwmonio