strerror: relax test for Solaris

strerror: relax test for Solaris

Solaris returns "Error 0" for 0 vs. "Unknown error" for -1; while that
does not quite have the positive connotation that POSIX recommends, it
is distinct enough that we do not need to replace the native strerror
just to pick a better string.

The test is really trying to reject implementations that use the same
string for both 0 and 1 (modulo any %d output of the error number).

* tests/test-strerror.c (main): Permit Solaris behavior.
* tests/test-strerror_r.c (main): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>