test: database_test: do_with_some_data: retain tmpdir for test duration
test: database_test: do_with_some_data: retain tmpdir for test duration

Currently, the test seems to use the tmpdir class in a wrong way,
just to get a path to a temporary directory.

It should keep the tmpdir object around for the duration of the test
so the temporary directory will be automatically removed when the test
completes.

Refs #6344

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20200504153810.202218-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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