self tests: register an expected failures with Solaris /bin/sh

self tests: register an expected failures with Solaris /bin/sh

Solaris 10 /bin/sh erroneously exit with success right away when
the following three conditions are met at the same time:
 1. the `errexit' flag is active,
 2. an exit trap is installed, and
 3. a non-existing command is issued.

* tests/self-exit.tap: When that bug is detected, issue an XFAIL
rather than a FAIL.  Since we are at it, improve by avoiding
creation of stray temporary files when testing for non-executable
commands.