Print the list of open file descriptors if exec-leak-check fails

This commit also adds a new environment variable TEST_DEBUG_FD_LEAK_CHECK.
If it is set, the test will also print the open file descriptors before
and after exec.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
3 jobs for fd-leak-check-debug-output in 1 minute and 1 second (queued for 1 second)
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debian:buster@container-prep

00:00:29

 
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00:00:20