Make testUtf8StringLiterals work when char is unsigned

...as is reportedly the case for Linux AArch64

Change-Id: I7e11c42f4437c8aad9dd734603fa7e0d458c9754

sal/qa/rtl/strings/test_ostring.cxx

@@ -111,9 +111,9 @@ void Test::testUtf8StringLiterals()
 {
     const OString sIn(u8"ßa");
     CPPUNIT_ASSERT_EQUAL(static_cast<sal_Int32>(3), sIn.getLength());
-    CPPUNIT_ASSERT_EQUAL(-61, static_cast<int>(sIn[0]));
-    CPPUNIT_ASSERT_EQUAL(-97, static_cast<int>(sIn[1]));
-    CPPUNIT_ASSERT_EQUAL(97, static_cast<int>(sIn[2]));
+    CPPUNIT_ASSERT_EQUAL(195, int(static_cast<unsigned char>(sIn[0])));
+    CPPUNIT_ASSERT_EQUAL(159, int(static_cast<unsigned char>(sIn[1])));
+    CPPUNIT_ASSERT_EQUAL(97, int(static_cast<unsigned char>(sIn[2])));
 }
 
 CPPUNIT_TEST_SUITE_REGISTRATION(Test);