testing: add a utility function to wait for file create
This is similar to `tests/testlib/wait-on-file`, but for the python code
Differential Revision: https://phab.mercurial-scm.org/D10476
streamclone: check the errors log in tests
Code can be buggy, in that case having error message tend to help.
Differential Revision: https://phab.mercurial-scm.org/D10475