Tests: Do not remove interrupted test log files by default

Tests: Do not remove interrupted test log files by default

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

During CI runs, builds may timeout or be killed for another reason.
Those tests logs are deleted and cannot be checked for diagnostic
information, warnings, or errors.

Cause
=====

By default, the test log for the currently running test is deleted by
automake so that subsequent invocations of `make check` will re-run the test.

Solution
========

Add a disable flag `--disable-precious-tests` and set
`PRECIOUS_TESTS` to true by default when configuring lttng-tools. When
`PRECIOUS_TOOLS` is set, all test logs in `tests/regression` will be
marked as `.PRECIOUS` and subsequently not deleted when interrupted.

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

This could make interrupting a test and re-running during test
development more of a hassle.

References
==========

[1]: https://www.gnu.org/software/make/manual/html_node/Special-Targets.html
[2]: https://automake.gnu.narkive.com/1TjEGbH2/delete-on-error-test-suite-log-and-precious

Change-Id: I08b4a1bb29eb609827cc1c047f141f94b210effe
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>