combined_mutation_reader_test: use the global const objects directly
combined_mutation_reader_test: use the global const objects directly

Instead of local ones.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <3ec1a70e4c0198c0563dff9688bbaa7fcfcace71.1502891190.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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