selftests/breakpoints: Fix format specifier in ksft_print_msg in step_after_suspend_test.c

In the function 'tools/testing/selftests/breakpoints/run_test' within
step_after_suspend_test.c, the ksft_print_msg function call incorrectly
used '$s' as a format specifier. This commit corrects this typo to use the
proper '%s' format specifier, ensuring the error message from
waitpid() is correctly displayed.

The issue manifested as a compilation warning (too many arguments
for format [-Wformat-extra-args]), potentially obscuring actual
runtime errors and complicating debugging processes.

This fix enhances the clarity of error messages during test failures
and ensures compliance with standard C format string conventions.

Signed-off-by: angquan yu <angquan21@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>