PEL: Create class to read from the journal

Create a Journal class that can extract messages out of the journal and
return them as a vector of strings that look like:

"Dec 14 15:58:17 systemd[1]: systemd-tmpfiles-clean.service: Deactivated
successfully."

It can either grab the previous N entries, or the previous N entries
that match a specific SYSLOG_IDENTIFIER value.

The class follows the same strategy as the DataInterface class where a
base class pointer is passed into the PEL Manager class so that during
unit test it can be mocked.

Future commits will capture the journal into PEL UserData sections.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I9f4bb304c4b213165049fa00de2e62f962ae67f1
diff --git a/test/openpower-pels/mocks.hpp b/test/openpower-pels/mocks.hpp
index de7f757..7a1ee35 100644
--- a/test/openpower-pels/mocks.hpp
+++ b/test/openpower-pels/mocks.hpp
@@ -1,5 +1,6 @@
 #include "extensions/openpower-pels/data_interface.hpp"
 #include "extensions/openpower-pels/host_interface.hpp"
+#include "extensions/openpower-pels/journal.hpp"
 
 #include <fcntl.h>
 
@@ -278,5 +279,14 @@
     size_t _cmdsProcessed = 0;
 };
 
+class MockJournal : public JournalBase
+{
+  public:
+    MockJournal() {}
+
+    MOCK_METHOD(std::vector<std::string>, getMessages,
+                (const std::string&, size_t), (const override));
+};
+
 } // namespace pels
 } // namespace openpower