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>

tools/testing/selftests/breakpoints/step_after_suspend_test.c

@@ -89,7 +89,7 @@ int run_test(int cpu)
 
 	wpid = waitpid(pid, &status, __WALL);
 	if (wpid != pid) {
-		ksft_print_msg("waitpid() failed: $s\n", strerror(errno));
+		ksft_print_msg("waitpid() failed: %s\n", strerror(errno));
 		return KSFT_FAIL;
 	}
 	if (WIFEXITED(status)) {