tests: assign to `value` property in test_assign_str/test_assign_vstr

tests: assign to `value` property in test_assign_str/test_assign_vstr

These tests don't do what was initially intended.  The intent is to
assign a string to the value in `self._def`.  Instead, they overwrite
`self._def`.  Fix that by assigning to the `value` property.

Change-Id: Ic5d0e797d5da7cc0fb17daa0b5e221db5c440bf9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11446
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>