tests: presenters: skip html_visuals test if 'sng' binary is not available

The test_html_visuals tries to compare an output generated by sng
package to a string. When 'sng' is not available, this test falls back
to binary comparison and thus, not matching the given string. This
patch adds the presence of 'sng' as condition to run the test.

tests/test_presenters.py

@@ -115,7 +115,7 @@ def test_html_option_with_file(tmpdir, capsys):
         body = extract_body(f.read())
         assert body.count('div class="difference"') == 4
 
-@skip_unless_tools_exist('compare', 'convert')
+@skip_unless_tools_exist('compare', 'convert', 'sng')
 def test_html_visuals(tmpdir, capsys):
     report_path = str(tmpdir.join('report.html'))