bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)

- bugfix and test for fragile metavar handling in argparse (see
  bpo-24089, bpo-14046, bpo-25058, bpo-11874)
- also fixes some incorrect tests that did not make 1-element tuples correctly

Lib/argparse.py

@@ -327,7 +327,11 @@ class HelpFormatter(object):
             if len(prefix) + len(usage) > text_width:
 
                 # break usage into wrappable parts
-                part_regexp = r'\(.*?\)+|\[.*?\]+|\S+'
+                part_regexp = (
+                    r'\(.*?\)+(?=\s|$)|'
+                    r'\[.*?\]+(?=\s|$)|'
+                    r'\S+'
+                )
                 opt_usage = format(optionals, groups)
                 pos_usage = format(positionals, groups)
                 opt_parts = _re.findall(part_regexp, opt_usage)