test/crimson: use std::for_each() for applying plain function

test/crimson: use std::for_each() for applying plain function

let's just use std::for_each() for applying function not returning future
to a range, otherwise we'd have

../src/test/crimson/seastore/test_transaction_manager.cc:950:5: error: no matching function for call to 'parallel_for_each'
    seastar::parallel_for_each(
    ^~~~~~~~~~~~~~~~~~~~~~~~~~
../src/seastar/include/seastar/core/loop.hh:559:1: note: candidate template ignored: constraints not satisfied [with Iterator = boost::iterators::counting_iterator<unsigned int, boost::use_default, boost::use_default>, Sentinel = boost::iterators::counting_iterator<unsigned int, boost::use_default, boost::use_default>, Func = (lambda at ../src/test/crimson/seastore/test_transaction_manager.cc:953:7)]
parallel_for_each(Iterator begin, Sentinel end, Func&& func) noexcept {
^
../src/seastar/include/seastar/core/loop.hh:553:78: note: because type constraint 'std::same_as<void, future<> >' was not satisfied:

Signed-off-by: Kefu Chai <tchaikov@gmail.com>