message: shorten message type

Create an alias `sdbusplus::message_t` to `sdbusplus::message::message`
to reduce duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib400b12fe4a412c0c0c3d26a88f3fae46445cfa8
diff --git a/test/message/read.cpp b/test/message/read.cpp
index d1d78e7..6131c24 100644
--- a/test/message/read.cpp
+++ b/test/message/read.cpp
@@ -43,7 +43,7 @@
             .WillRepeatedly(Return(0));
     };
 
-    sdbusplus::message::message new_message()
+    sdbusplus::message_t new_message()
     {
         return sdbusplus::get_mocked_new(&mock).new_method_call(
             nullptr, nullptr, nullptr, nullptr);