test/multishard_mutation_query_test: eliminate another unsafely used boost test macro
test/multishard_mutation_query_test: eliminate another unsafely used boost test macro

Boost test macros are not thread safe, using them from multiple threads
results in garbled XML test report output.
3f1823a4f096b855d63d5b18bb7a12593c22e0fa replaced most of the
thread-unsafe boost test macros in multishard_mutation_query_test, but
one still managed to slip through the cracks. This patch removes that as
well.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200529130706.149603-3-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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