selftests: breakpoint_test: use ksft_* var arg msg api

Use ksft_* var arg msg to include strerror() info. in test output. Change
output from child process to use ksft_print_msg() instead of ksft_exit_*
to avoid double counting tests and ensure parent does the incrementing
test counters. Also includes unused variable cleanup.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>