tests: fix error message check on some systems

http://hydra.nixos.org/build/18129583 identified (on OS X)
an incorrect test assumption in the previous commit.

* gl/lib/xdectoint.c (__xnumtoint): Suppress the EINVAL
error message as it's redundant in this context.
* tests/misc/tail.pl: Suppress _optionally_ appended
strerror messages.
* tests/fmt/base.pl: Likewise.
* tests/pr/pr-tests.pl: Likewise.
* tests/split/l-chunk.sh: Likewise.