Move getMessage and getMessageFromRegistry to cpp and add test

This change moves getMessage and getMessageFromRegistry to a .cpp file
so that they can be easily tested.

Tested: Unit test passes
Signed-off-by: Sui Chen <suichen@google.com>
Change-Id: Ia9fc91e5a47036198bf013ff3ea21ea9f6d5259a
diff --git a/meson.build b/meson.build
index ae789a9..93ac8da 100644
--- a/meson.build
+++ b/meson.build
@@ -327,6 +327,7 @@
 
 srcfiles_bmcweb = files(
   'redfish-core/src/error_messages.cpp',
+  'redfish-core/src/registries.cpp',
   'redfish-core/src/utils/json_utils.cpp',
   'src/boost_asio_ssl.cpp',
   'src/boost_asio.cpp',