Skip test_open() and test_os_open(): both tests uses a FIFO and signals, but
there is a bug in the FreeBSD kernel which blocks the test. Skip the tests
until the bug is fixed in FreeBSD kernel.

Remove also debug traces from test_eintr:

* stop using faulthandler to have a timeout
* remove print()

Write also open and close on two lines in test_open() and test_os_open()
tests. If these tests block again, we can know if the test is stuck at open or
close.

test_eintr: don't always run the test in debug mode.