Add test: no clock class in trace's packet header field type

Add test: no clock class in trace's packet header field type

This patch adds a test case which uses the Python bindings to test that
it is not possible to set a trace's packet header field type containing
an integer field type mapped to a clock class.

The new tests/lib/ctf-ir directory is set so that future CTF IR test
cases can be added easily if need be instead of going to
tests/lib/test_ctf_writer.c.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>