Tests: Make notifier discard count test more robust

Tests: Make notifier discard count test more robust

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

In the CI, this test would intermittently fail. During failures,
the calculated pipe size from the `default_pipe_size_getter`
application was 8192, while in other cases it was 65536.

```
ERROR: tools/notification/test_notification_notifier_discarded_count
====================================================================

1..41
ok 1 - Add trigger my_trigger
PASS: tools/notification/test_notification_notifier_discarded_count 1 - Add trigger my_trigger
  ---
    duration_ms: 1323.966137
  ...
ok 2 - No discarded tracer notification
PASS: tools/notification/test_notification_notifier_discarded_count 2 - No discarded tracer notification
  ---
    duration_ms: 22.021590
  ...
ok 3 - Generating 390 tracer notifications
PASS: tools/notification/test_notification_notifier_discarded_count 3 - Generating 390 tracer notifications
  ---
    duration_ms: 154.790871
  ...
not ok 4 - Discarded tracer notification number non-zero (0) as expected
FAIL: tools/notification/test_notification_notifier_discarded_count 4 - Discarded tracer notification number non-zero (0) as expected
  ---
    duration_ms: 24.323759
  ...
```

Cause
=====

The initial size of pipes in linux may have different values:

1) `16 * PAGE_SIZE` (as documented in `man 7 pipe`) (since Linux 2.6.11)
2) When a user has many pipes open and is above a soft limit:

  * `2 * PAGE_SIZE` (undocumented, see[1]), as of Linux 5.14[2]
  * `1 * PAGE_SIZE` since linux 2.6.35[3]

As the program `default_pipe_size_getter` opened a pipe to check it's
size, there could be times in a system where a user has many pipe
buffers open beyond the soft limit and the lower value would be
returned; however, the previously opened sessiond may have had a pipe
opened with the larger default pipe size.

Solution
========

Use the maximum page size (on Linux, from
`/proc/sys/fs/pipe-max-size`) for the estimated pipe size rather than
opening a pipe and checking it's size.

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

When the maximum pipe size value is much larger than the actual size
of the notification pipe, many more events are emitted than is
necessary to complete the test.

References
==========

[1]: https://gitlab.com/linux-kernel/stable/-/blob/3e9bff3bbe1355805de919f688bef4baefbfd436/fs/pipe.c#L809
[2]: See upstream commit 46c4c9d1beb7f5b4cec4dd90e7728720583ee348
[3]: See upstream commit 6a6ca57de92fcae34603551ac944aa74758c30d4

Change-Id: Id547a1d772b5a7f9b18ffa686ff6644afca4ab15
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>