Fix: test_ir_visit.c: create valid SC PC/EH and trace packet header

Fix: test_ir_visit.c: create valid SC PC/EH and trace packet header

Since CTF IR is more strict regarding the semantics of trace packet
header, stream class packet context and event header field types, this
test fails because the default packet context field type of a stream
class which is not created for a CTF writer object contains
`timestamp_begin` and `timestamp_end` fields which are not mapped to a
clock class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>