Fix: tests: fix unused-but-set warning in test_fd_tracker.c

Fix: tests: fix unused-but-set warning in test_fd_tracker.c

When building with clang-14 on Ubuntu 20.04, I get:

      CC       test_fd_tracker.o
    /home/smarchi/src/lttng-tools/tests/unit/test_fd_tracker.c:169:15: error: variable 'fds_set_to_minus_1' set but not used [-Werror,-Wunused-but-set-variable]
            unsigned int fds_set_to_minus_1 = 0;
                         ^

The compiler seems right, so remove fds_set_to_minus_1.  It might be
that the intention was to assert something using this variable, but I
couldn't figure it out.

Change-Id: I12bfd07bca7829de8d5b85d375d9b52bd84d677a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>