tests: Make test_per_application_leaks more robust

tests: Make test_per_application_leaks more robust

Observed issue
==============

On some systems (e.g. sles15sp4, RHEL 7), test_per_application_leaks
would fail spuriously.

Cause
=====

When walking the the FDs in `/proc/XX/fds`, the symbolic links exist but
link to unlinked files. E.g.

```
lrwx------ 1 root root 64 May 22 14:49 /proc/83578/fd/58 -> '/dev/shm/shm-ust-consumer-83578 (deleted)'
```

Solution
========

The test has been modified to loop, waiting for the post-destroy shm
count to drop back to the post-start count. In the case of a failure,
the test will hang forever but doesn't fail spuriously.

Known drawbacks
===============

None.

Change-Id: Id3c8a9f6db83fe888e79b8f06cb8308b4d90da87
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>