gtester-report: fix range usage when running as python3 app

When using python3 as interpreter, range only takes integer arguments or
it results in errors like:

   File "/usr/bin/gtester-report", line 78, in html_indent_string
     for i in range (0, (n + 1) / 2):
 TypeError: 'float' object cannot be interpreted as an integer

https://bugzilla.gnome.org/show_bug.cgi?id=791296

glib/gtester-report

@@ -75,7 +75,7 @@ def attribute_as_text (node, aname, node_name = None):
 def html_indent_string (n):
   uncollapsible_space = '  ' # HTML won't compress alternating sequences of ' ' and ' '
   string = ''
-  for i in range (0, (n + 1) / 2):
+  for i in range (0, int((n + 1) / 2)):
     string += uncollapsible_space
   return string