tests: remove obsolete uses of $sh_errexit_works

tests: remove obsolete uses of $sh_errexit_works

After the last changes, configure will ensure that the shell
selected to run the test scripts can correctly propagate exit
status to the exit trap when 'set -e' is in effect.

* configure.ac (sh_errexit_works): Do not AC_SUBST it anymore.
* defs-static.in (sh_errexit_works): Do not initialize
from the AC_SUBST value anymore.
* defs (trap): Trap the EXIT signal unconditionally.
* t/self-check-explicit-skips.sh: Do not skip the test if
'$sh_errexit_works' is != "yes", this check doesn't make
sense anymore.
* t/self-check-cleanup.tap: Likewise.
* t/self-check-exit.tap: Assume the exit trap is always
installed by ./defs.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>