test: Use appropriate OUString functions on string constants

Change-Id: Idbdc5c9aab8fa65df39d66da2461337db6350e52

test/source/bootstrapfixture.cxx

@@ -173,7 +173,7 @@ void test::BootstrapFixture::validate(const OUString& rPath, test::ValidationFor
 
     int returnValue = system(OUStringToOString(aCommand, RTL_TEXTENCODING_UTF8).getStr());
     CPPUNIT_ASSERT_EQUAL_MESSAGE(
-        OUStringToOString(OUString("failed to execute: ") + aCommand,
+        OUStringToOString("failed to execute: " + aCommand,
             RTL_TEXTENCODING_UTF8).getStr(), 0, returnValue);
 
     OString aContentString = loadFile(aOutput.GetURL());