hghave: disallow symlinks on Windows

Symlinks on Windows require either a special priviledge, or enabling Developer
Mode.  It's probably the latter that is enabled on the new CI machine.  But
since Mercurial itself is saying no to symlinks on Windows, the tests for
symlinks shouldn't be attempted.  This should fix a lot of the noise in the py3
tests.

Differential Revision: https://phab.mercurial-scm.org/D7233

tests/hghave.py

@@ -431,7 +431,8 b' def has_p4():'
 
 @check("symlink", "symbolic links")
 def has_symlink():
-    if getattr(os, "symlink", None) is None:
+    # mercurial.windows.checklink() is a hard 'no' at the moment
+    if os.name == 'nt' or getattr(os, "symlink", None) is None:
         return False
     name = tempfile.mktemp(dir='.', prefix=tempprefix)
     try: