Add tracing tests

[igniteflow]

Adds tests for xocto.tracing

Before

> pytest --cov=xocto

Name                          Stmts   Miss  Cover
-------------------------------------------------
...
xocto/tracing.py                 16     16     0%
...

After

> pytest --cov=xocto

Name                          Stmts   Miss  Cover
-------------------------------------------------
...
xocto/tracing.py                 16     0     100%
...

Note that I haven't used pytest much before. I feel like the first two lines of each test could be encapsulated in the pytest equivalent for setUp() - perhaps setup_method?