Updating test case

gdb.arch/aarch64-dbreg-contents.exp:
 * Replaced "run" with "runto_main + continue".
 * Replaced "gdb_compile + clean_restart" with "prepare_for_testing".
 * Added comment for case "exited with code 01".

gdb.arch/aarch64-dbreg-contents.c:
 * Removed SET_WATCHPOINT marco.
 * Removed redundent cleanup ().
 * Cleaned up comment.
3 files changed