Use a specific directory to nest keyspace used during test runs

[vedang]

Currently, tests create tuples with a random prefix, and cleanup all tuples starting with the prefix on completion.

Once directories are in place, all keys created in a testrun should be created inside a specific subspace inside a test directory. Modify the test-fixture in src/clj_fdb/internal/util.clj to use a specific directory (say testcycle)

This task is blocked by #14