py3: string normalization and I/O tweaks in test-lfs.t

The print was inserting b'' on Python 3. In addition, since we
weren't writing to the ui instance (which isn't readily available
in this function), output order could get mixed up.

We add some pycompat casts and a stdout flush to make the test
happy on all Python versions.

Differential Revision: https://phab.mercurial-scm.org/D7938

tests/test-lfs.t

@@ -810,8 +810,11 b' The extension makes sure that the filelo'
 blob, and the output shows that it isn't fetched.
 
   $ cat > $TESTTMP/lfsrename.py <<EOF
+  > import sys
+  > 
   > from mercurial import (
   >     exthelper,
+  >     pycompat,
   > )
   > 
   > from hgext.lfs import (
@@ -829,7 +832,10 b" blob, and the output shows that it isn't"
   >         rawtext = self._revlog.rawdata(node)
   >         metadata = pointer.deserialize(rawtext)
   >         print('lfs blob %s renamed %s -> %s'
-  >               % (metadata[b'oid'], ret[0], self._revlog.filename))
+  >               % (pycompat.sysstr(metadata[b'oid']),
+  >                  pycompat.sysstr(ret[0]),
+  >                  pycompat.fsdecode(self._revlog.filename)))
+  >         sys.stdout.flush()
   >     return ret
   > EOF