test: Make multishard_mutation_query test do less scans
test: Make multishard_mutation_query test do less scans

When built by clang this dev-mode test takes ~30 minutes to
complete. Let's reduce this time by reducing the scale of
the test if DEVEL is set.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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