PostgresNode already retained base directories in such cases.  Stop
using $SIG{__DIE__}, which is redundant with the exit status check, in
lieu of proliferating it to TestLib.  Back-patch to 9.6, where commit
88802e06 introduced retention on
failure.

Reviewed by Daniel Gustafsson.

Discussion: https://postgr.es/m/20200202170155.GA3264196@rfd.leadboat.com