test/boost/mutation_reader_test: share permit between readers in a read
test/boost/mutation_reader_test: share permit between readers in a read

Permits were designed such that there is one permit per read, being
shared by all readers in that read. Make sure readers created by tests
adhere to this.

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