bus: shorten bus type

Create an alias `sdbusplus::bus_t` to `sdbusplus::bus::bus` to reduce
duplication.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9f17892eed5112d49469498614884c02bba86011
diff --git a/test/server/message_variant.cpp b/test/server/message_variant.cpp
index 0fcde8d..2ddfac4 100644
--- a/test/server/message_variant.cpp
+++ b/test/server/message_variant.cpp
@@ -8,7 +8,7 @@
 
 struct Object : public ::testing::Test
 {
-    sdbusplus::bus::bus bus = sdbusplus::bus::new_default();
+    sdbusplus::bus_t bus = sdbusplus::bus::new_default();
     sdbusplus::message_t msg = bus.new_method_call(
         "xyz.openbmc_project.sdbusplus.test.Object",
         "/xyz/openbmc_project/sdbusplus/test/object",