tests: Rework test suite

tests: Rework test suite

  * Test script are not generated by autoconf anymore, instead the
    parameters are passed trought a set of environment variable prefixed
    with BT_TESTS_. This will enable the test suite to be used against
    a system installed version of babeltrace with minimal configuration.
  * Move all duplicated shell script code to 'utils.sh'.
  * Add 'set -u' to common.sh to catch undefined variables in test
    scripts. Variables that are expected to be undefined must be used as
    '${var:-}'.
  * All shell script is shellcheck tested.
  * Removed unused test 'test_trace_ir'
  * The python test runner now returns an error when no python test
    scripts are found.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I15dcda1d147c8c72c6e4ee00052591eb6f9e30bd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1429
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>