Clean-up: tests: unchecked closedir value in test_fd_tracker

closedir's return value is not checked in a utility function
of the test_fd_tracker unit test.

1407702 Unchecked return value
If the function returns an error value, the error value may be mistaken for a normal value.
In fd_count: Value returned from a function is not checked for errors before being used (CWE-252)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>