heredoctest: do not append extra newline character to continuation line

Trailing newline characters are kept in lines.

tests/heredoctest.py

@@ -10,7 +10,7 b' while lines:'
         snippet = l[4:]
         while lines and lines[0].startswith('... '):
             l = lines.pop(0)
-            snippet += "\n" + l[4:]
+            snippet += l[4:]
         c = compile(snippet, '<heredoc>', 'single')
         try:
             exec c in globalvars

tests/test-unified-test.t

@@ -42,6 +42,10 b' Doctest commands:'
   ...     print foo, 'should be visible in func()'
   >>> func()
   global name should be visible in func()
+  >>> print '''multiline
+  ... string'''
+  multiline
+  string
 
 Regular expressions: