* test(Executors): Added tests for busy waiting

Checks if executors are busy waiting while they should block
in spin_some or spin_all.

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>

* fix: Reworked spinAll test

This test was strange. It looked like, it assumed that spin_all did
not return instantly. Also it was racy, as the thread could terminate
instantly.

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>

* fix(Executor): Fixed spin_all not returning instantly is no work was available

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>

* Update rclcpp/test/rclcpp/executors/test_executors.cpp

Co-authored-by: Tomoya Fujita <Tomoya.Fujita@sony.com>
Signed-off-by: jmachowinski <jmachowinski@users.noreply.github.com>

* test(executors): Added test for busy waiting while calling spin

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>

* fix(executor): Reset wait_result on every call to spin_some_impl

Before, the method would not recollect available work in case of
spin_some, spin_all. This would lead to the method behaving differently
than to what the documentation states.

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>

* restore previous test logic for now

Signed-off-by: William Woodall <william@osrfoundation.org>

* refactor spin_some_impl's logic and improve busy wait tests

Signed-off-by: William Woodall <william@osrfoundation.org>

* added some more comments about the implementation

Signed-off-by: William Woodall <william@osrfoundation.org>

---------

Signed-off-by: Janosch Machowinski <J.Machowinski@cellumation.com>
Signed-off-by: jmachowinski <jmachowinski@users.noreply.github.com>
Signed-off-by: William Woodall <william@osrfoundation.org>
Co-authored-by: Janosch Machowinski <J.Machowinski@cellumation.com>
Co-authored-by: jmachowinski <jmachowinski@users.noreply.github.com>
Co-authored-by: Tomoya Fujita <Tomoya.Fujita@sony.com>
(cherry picked from commit f7185dc)