test: fstream_test: adjust fstream_wrapper constructor for C++20
test: fstream_test: adjust fstream_wrapper constructor for C++20

C++20 changed the rules around bracket initialization, so adjust
fstream_wrapper to fit. The adjustment is traditional - add a
constructor and call it with a regular parenthesized constructor
call.

avikivity/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,scylladb/seastar