Set the minWidth and the minHeight of the endPanel created for a box layout

[koendehondt]

@estebanlm

The PR for Pharo 12 was #1507.

This is the PR to fix the same problem in Pharo 11.

[koendehondt]

The changes made two box layout tests fail, because of rounding errors on the width of the children of the box layout. I adapted the code to avoid rounding errors.
On CI, the initial window width is 240 pixels instead of the 400 pixels in SpWindowPresenter>>#defaultInitialExtent. That made the tests fail on CI, while they were green locally. That issue accounts for the many commits to adapt the tests.
The tests used a list, but its initial width is bigger than the space allotted by the box layout. That is why some assertions used a >= instead of =. I replaced the list by a button.
I wonder whether the previous issue was the reason to add #skipOnPharoCITestingEnvironment in two tests. I took them out, as the tests are green on CI now.

Btw: does #skipOnPharoCITestingEnvironment actually avoid running a test on CI? They were run and they failed, so it is probably a good idea to check whether the environment variable is in use on CI.

[koendehondt]

@estebanlm The tests are green except for one that does not seem to be related to the changes. Please review and merge.