tests: mutation_fragment_test: de-template
tests: mutation_fragment_test: de-template

The for_each_target() template is called four times, so making it a normal function
reduces a lot of code generation.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla