test: lister_test: test_lister_abort: generate at least one entry
test: lister_test: test_lister_abort: generate at least one entry

Without this fix, generate_random_content could generate 0 entries
and the expected exception would never be injected.

With it, we generate at least 1 entry and the test passes
with the offending random-seed:

```
random-seed=1898914316
Generated 1 dir entries
Aborting lister after 1 dir entries
test/boost/lister_test.cc(96): info: check 'exception "expected_exception" raised as expected' has passed
```

Fixes #9953

Test: lister_test.test_lister_abort --random-seed=1898914316(dev)

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220123122921.14017-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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