tests: use readlink.py instead of readlink

The latter doesn't always exist, such as on Solaris-derived platforms.

Differential Revision: https://phab.mercurial-scm.org/D998

tests/test-pathconflicts-merge.t

@@ -121,8 +121,8 b' Merge - local directory conflicts with r'
   use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
   [1]
   $ hg mv a/b~2ea68033e3be a/b.old
-  $ readlink a/b.old
-  c
+  $ readlink.py a/b.old
+  a/b.old -> c
   $ hg resolve --mark a/b
   (no more unresolved files)
   $ hg commit -m "merge link (rename link)"