debuggerd_test: fix crasher path.

https://android-review.googlesource.com/#/c/331200 moved crasher to
using soong, which changed its location from /system/xbin/crasher to
/system/bin/crasher.

Bug: http://b/35100742
Test: /data/nativetest/debuggerd_test/debuggerd_test32
Test: /data/nativetest64/debuggerd_test/debuggerd_test64
Change-Id: I16a2050b257277023773cc0c960b5ab36e0c7cd4