tests: avoid false failure due to mismatched isblank()
There is a mismatch between isblank() used by tr and c32isblank() now
used by uniq on Solaris 11 OpenIndiana. isblank() was seen to return
true for non breaking space, while c32isblank() returned false.
Interestingly on Solaris, non breaking space is considered a blank
character, and isblank() and c32isblank() honor this in all locales.
* tests/uniq/uniq.pl: Adjust the blank check to use join(1) rather than
tr(1), as join uses the same blank determination routines as uniq(1).