CI: show skipped Python tests

Call pytest3 with argument -ra to display the reason why Python tests are
skipped.

The -r flag displays a test summary info for each test. -ra eliminates
this info for passed tests.

Pros an cons were discussed in:
https://lists.denx.de/pipermail/u-boot/2020-June/417090.html



Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>