Fixup test-suite for ubuntu-20.04 image

[Farigh]

The ubuntu-20.04 version of the github image contains a lot of tools updates (valgrind 3.15, glibc 2.31, ...)

The tests do not pass in this version, the image was changed from ubuntu-latest to ubuntu-18.04 for now, as a workaround.
(see commit ab8e764)

Any environment version should work. The test-suite should be reworked in order to achieve this.