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/message/call.cpp b/test/message/call.cpp
index 0ef015f..f0122ee 100644
--- a/test/message/call.cpp
+++ b/test/message/call.cpp
@@ -20,13 +20,13 @@
     m.read(globalId);
 }
 
-message newBusIdReq(bus::bus& b)
+message newBusIdReq(bus_t& b)
 {
     return b.new_method_call("org.freedesktop.DBus", "/org/freedesktop/DBus",
                              "org.freedesktop.DBus", "GetId");
 }
 
-std::string syncBusId(bus::bus& b)
+std::string syncBusId(bus_t& b)
 {
     std::string ret;
     newBusIdReq(b).call().read(ret);