Fix: scan-build warnings in CTF Writer/IR tests

scan-build reported that strcmp could be called on a NULL string
when some tests fail.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add UUID accessors to CTF-IR clock

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tests: Add tests for new clock UUID accessors

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>