Fix xsheetfilterable CppUnit assert

Change to Logical NOT operator to make
clang loplugin:simplifybool happy.

Change-Id: I95af212ea6fa33be2775824322bd64f04d2ac695
Reviewed-on: https://gerrit.libreoffice.org/44039Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>

test/source/sheet/xsheetfilterable.cxx

@@ -80,7 +80,7 @@ void XSheetFilterable::testFilter()
     bool bIsVisible = true;
     CPPUNIT_ASSERT(xRowProps->getPropertyValue("IsVisible") >>= bIsVisible);
 
-    CPPUNIT_ASSERT_MESSAGE("Row 1 should be invisible", bIsVisible == false);
+    CPPUNIT_ASSERT_MESSAGE("Row 1 should be invisible", !bIsVisible);
 }
 
 }