Add a single generated env file to the test suite

Add a single generated env file to the test suite

Regroup all the configure detected values relevant to the test suite in
a single generated file. This file will be automatically sourced by the
test suite in most scenarios but can also be sourced in the shell of a
user.

 * All user overridable variables start with 'BT_TESTS_'.
 * The priority for variables is :
     Environment -> env.sh -> utils.sh (defaults).
 * A user can source 'env.sh', override some of the values and manually
   run test scripts.
 * The test suite can run without an 'env.sh' file present.

Scenarios:

 * Manually running tests in an in-tree build:

   The test suite sources 'env.sh' relative to itself, no environment
   variables required.

 * Manually running tests in an out-of-tree build:

   The user sets BT_TESTS_BUILDDIR in its environment or sources 'env.sh'.

 * Running the test suite with 'make check' in either builds:

   The Makefile sets BT_TESTS_BUILDDIR in the environment which allows
   the test suite to source env.sh in either in-tree or out-of-tree builds.

Change-Id: I1ba3775ea3d724345cab769af9b2efc22ee53585
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6314
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>