changed_predicate_spec: ensure target directory exists (#1305)
changed_predicate_spec: ensure target directory exists (#1305)

When the tests on the Debian package, this test failed because ./tmp/
./tmp/ didn't exist. Instead of just assuming ./tmp/ exists, write to a
dynamically generated a dynamically generated temporary directory
instead.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick