UnitTests: Disable warning for self assign overloaded

We want to test it so we cannot fix the code.

Change-Id: Ieb0517e539ae60c275069b790ca32380d39a59f0
Reviewed-by: Vikas Pachdha <vikas.pachdha@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/smallstring-test.cpp

@@ -1609,6 +1609,11 @@ TEST(SmallString, LongSmallStringCopyAssignment)
     ASSERT_THAT(copy, text);
 }
 
+#if __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wself-assign-overloaded"
+#endif
+
 TEST(SmallString, LongSmallStringCopySelfAssignment)
 {
     SmallString text("this is a very very very very long text");
@@ -1618,6 +1623,10 @@ TEST(SmallString, LongSmallStringCopySelfAssignment)
     ASSERT_THAT(text, SmallString("this is a very very very very long text"));
 }
 
+#if __clang__
+#pragma clang diagnostic pop
+#endif
+
 TEST(SmallString, ShortSmallStringMoveAssignment)
 {
     SmallString text("text");