test: sstable_datafile_test: tombstone_purge_test: use a longer ttl
test: sstable_datafile_test: tombstone_purge_test: use a longer ttl

As seen in next-3319 unit testing on jenkins
The cell ttl may expire during the test (presuming
that the test machine was overloaded), leading to:
```
INFO  2021-03-21 10:05:23,048 [shard 0] compaction - [Compact tests.tombstone_purge 2fcaf680-8a1c-11eb-b1b9-97020c5d261e] Compacting [/jenkins/workspace/scylla-master/next/scylla/testlog/release/scylla-af8644ec-7f07-4ffe-80bf-6703a942e435/la-17-big-Data.db:level=0:origin=, ]
INFO  2021-03-21 10:05:23,048 [shard 0] compaction - [Compact tests.tombstone_purge 2fcaf680-8a1c-11eb-b1b9-97020c5d261e] Compacted 1 sstables to []. 4kB to 0 bytes (~0% of original) in 0ms = 0 bytes/s. ~128 total partitions merged to 0.
./test/lib/mutation_assertions.hh(108): fatal error: in "tombstone_purge_test": Mutations differ, expected {table: 'tests.tombstone_purge', key: {'id': alpha, token: -7531858254489963}, mutation_partition: {
  rows: [
    {
      cont: true,
      dummy: false,
      position: {
        bound_weight: 0,
      },
      'value': { atomic_cell{1,ts=1616313953,expiry=1616313958,ttl=5} },
    },
  ]
}
}
 ...but got: {table: 'tests.tombstone_purge', key: {'id': alpha, token: -7531858254489963}, mutation_partition: {
  rows: [
    {
      cont: true,
      dummy: false,
      position: {
        bound_weight: 0,
      },
      'value': { atomic_cell{DEAD,ts=1616313953,deletion_time=1616313953} },
    },
  ]
}
}
```

This corresponds to:
```
2395            auto mut2 = make_expiring(alpha, ttl);
2396            auto mut3 = make_insert(beta);
...
2399            auto sst2 = make_sstable_containing(sst_gen, {mut2, mut3});
```

Extend (logical) ttl to 10 seconds to reduce flakiness
due to real-time timing.

Test: sstable_datafile_test(dev)
Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20210321142931.1226850-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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