UnoApiTest: simplify code by using tempfile member

SwModelTestBase does the same. this will help to make
SwModelTestBase inherit from UnoApiTest

Change-Id: If1c824cf92f0e8b70253e4d5fdeddcaa521d4632
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/142287
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>

cppcanvas/CppunitTest_cppcanvas_emfplus.mk

@@ -25,6 +25,7 @@ $(eval $(call gb_CppunitTest_use_libraries,cppcanvas_emfplus, \
 	test \
 	tl \
 	unotest \
+    utl \
 	vcl \
 ))