D-Bus testing: enable running under gdb via env variable

When TEST_DBUS_GDB is set to a non-empty value, syncevo-dbus-server is
run under gdb. Previously it was necessary to edit test-dbus.py to
achieve that.

test/test-dbus.py

@@ -48,7 +48,7 @@ except ImportError:
 
 DBusGMainLoop(set_as_default=True)
 
-debugger = "" # "gdb"
+debugger = os.environ.get("TEST_DBUS_GDB", None) and "gdb" or ""
 server = ["syncevo-dbus-server"]
 monitor = ["dbus-monitor"]
 # primarily for XDG files, but also other temporary files