PKGEXT is a makepkg variable referring to a fixed filename suffix, but
we were also using it to mean a bash glob referring to candidate
filenames. This is wrong, so rename it to PKGEXTS which is more
descriptive of its purpose.

Exclude the testsuite from this change, as the testsuite actually uses
PKGEXT for its intended purpose. Fix the testsuite to consistently use
PKGEXT, as it hardcoded the file extension in several cases, and pin
its value to .pkg.tar.xz