Fix: tests: conditionally skip Python bindings test that requires Python plugins...

Fix: tests: conditionally skip Python bindings test that requires Python plugins support

As exposed by bug 1272 [1], there are some tests in
test_trace_collection_message_iterator.py that depend on Python plugins
being enabled.  Running the tests with `--enable-python-bindings` and
`--disable-python-plugins` therefore leads to these tests failing.

Fix it by skipping them if the support for Python plugins is not
enabled.

[1] https://bugs.lttng.org/issues/1272

Change-Id: I4881a3b0a336b9078715837cb7f8f423bbc2b982
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3621
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
CI-Build: Michael Jeanson <mjeanson@efficios.com>