Small cleanups of debuggerd_test.

Don't try and add ugetrlimit for lp64 since that adds an
error from minijail.

When a signal mismatches, print the actual signal names.

Fix a bug trying to get the exit status from the wrong variable.

Test: All unit tests pass.
Change-Id: Iaf51037b6d5d6723d5b01d8bb91bb3593079a244