tests: constify format strings in tap.h

Format strings are typically read-only data (literal strings), and they
are not modified by the functions receiving them, so it makes no sense
for them not to be const.

This helps avoid some "passing value to function foo discards const
qualifier" kind of warnings.

Change-Id: Ibaaaf5690bed21e1d541f1b676627dfd70541219
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2130
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>