geli tests: Use shorter passphrases in setkey_passphrase

There is an undocumented limit on the length of a GELI passphrase, and
the test exceeded that. Most of the time it worked because a nul
terminator would appear early enough in the string.

Reported by: Jenkins
Fixes: 2b7b09ac9675 ("geli tests: Add a regression test for geli setkey -J")

(cherry picked from commit c4b927c341e23eea17f75d139aafcf58545c5b0e)