test-https: enable dummycert test only if Apple python is used (issue4500)

The dummycert trick works only if Python is linked to Apple's patched OpenSSL.

tests/hghave.py

@@ -320,6 +320,11 b' def has_ssl():'
     except ImportError:
         return False
 
+@check("defaultcacerts", "can verify SSL certs by system's CA certs store")
+def has_defaultcacerts():
+    from mercurial import sslutil
+    return sslutil._defaultcacerts()
+
 @check("windows", "Windows")
 def has_windows():
     return os.name == 'nt'

tests/test-https.t

@@ -119,7 +119,7 b' OS X has a dummy CA cert that enables us'
 Apple's OpenSSL. This trick do not work with plain OpenSSL.
 
   $ DISABLEOSXDUMMYCERT=
-#if osx
+#if defaultcacerts
   $ hg clone https://localhost:$HGPORT/ copy-pull
   abort: error: *certificate verify failed* (glob)
   [255]