tests: teach hghave to actually test for symlink support

tests/hghave

@@ -161,6 +161,15 b' def has_p4():'
     return matchoutput('p4 -V', r'Rev\. P4/') and matchoutput('p4d -V', r'Rev\. P4D/')
 
 def has_symlink():
+    if not hasattr(os, "symlink"):
+        return False
+    name = tempfile.mktemp(dir=".", prefix='hg-checklink-')
+    try:
+        os.symlink(".", name)
+        os.unlink(name)
+        return True
+    except (OSError, AttributeError):
+        return False
     return hasattr(os, "symlink") # FIXME: should also check file system and os
 
 def has_tla():