bpo-37935: Added tests for os.walk(), glob.iglob() and Path.glob() (GH-15956) (GH-16043)

Test that they do not keep too many file descriptors open for the host OS in a reasonable test scenario.

See [bpo-37935](https://bugs.python.org/issue37935).
(cherry picked from commit f9dc2ad89032201427ed5f08061c703794627ad9)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
3 files changed