test: reader_concurrency_semaphore_test: isolate from namespace of source_location
test: reader_concurrency_semaphore_test: isolate from namespace of source_location

More modern gcc uses std::source_location instead of
std::experimental::source_location. Rely on seastar::compat to get it
right for us.

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