[lit] Fix order of checks in shtest-shell.py test

Summary:
An expectation in `utils/lit/tests/Inputs/shtest-shell/redirects.txt`
expects that first a string printed to stdout is seen, and then a
string printed to stderr. Add `flush()` calls to ensure that stdout is
printed before stderr, as expected.

Reviewers: rnk, mgorny, jroelofs

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35947

llvm-svn: 309292