tests: remove redundant 'set -e' calls

tests: remove redundant 'set -e' calls

* t/aclocal-install-fail.sh: Do not set the 'errexit' shell flag,
as it is already set by './defs'.
* t/aclocal-install-mkdir.sh: Likewise.
* t/aclocal-no-install-no-mkdir.sh: Likewise.
* t/aclocal-verbose-install.sh: Likewise.
* t/instdir-no-empty.sh: Likewise.
* t/link_cond.sh: Likewise.
* t/python-pr10995.sh: Likewise.
* t/vala-vapi.sh: Likewise.

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