Plugins: skip the unimplemented test case instead of failing whole test.

It is better to run automatically all tests than to manually check that
only this one error is present.

src/Plugins/MagnumFont/Test/MagnumFontTest.cpp

@@ -98,6 +98,7 @@ void MagnumFontTest::layout() {
 
 void MagnumFontTest::createGlyphCache() {
     /** @todo */
+    CORRADE_SKIP("Not yet implemented");
 }
 
 }}}