tests: remove redundant settings of `errexit' shell flag

tests: remove redundant settings of `errexit' shell flag

* tests/aclocal-path-install-serial.test: Do not set the
`errexit' shell flag, as it is already set by `tests/defs'.
* tests/ar-lib2.test: Likewise.
* tests/ar-lib3.test: Likewise.
* tests/ar-lib4.test: Likewise.
* tests/ar-lib5a.test: Likewise.
* tests/ar-lib5b.test: Likewise.
* tests/ar-lib6a.test: Likewise.
* tests/ar-lib6b.test: Likewise.
* tests/ar-lib7.test: Likewise.
* tests/ar3.test: Likewise.
* tests/ar4.test: Likewise.
* tests/ar5.test: Likewise.
* tests/extra-portability.test: Likewise.
* tests/extra-portability2.test: Likewise.
* tests/extra-portability3.test: Likewise.
* tests/install-info-dir.test: Likewise.
* tests/maintmode-configure-msg.test: Likewise.