Making the tests themselves run on the strict set -euf

Previously, libraries were set up to run on set -euf, which
exists on any untreated error and unbound variable. This
commit makes the tests run on this setting as well.

Minor changes were needed in the sources:

  - depur_get_tracer needed variable defaults `${var:-}` for
    bash and ksh when the stack trace didn't included a file.
  - posit_external and dispatch now uses &&|| method to
    retrieve status code from unknown calls
  - Each test now includes lib/common.sh