test: network_topology_strategy_test: always stop_snitch after create_snitch
test: network_topology_strategy_test: always stop_snitch after create_snitch

Currently stop_snitch is not called if the test fails on exception.
This causes a failure in create_snitch where snitch_inst fails to start
since it wasn't stopped earlier.

For example:
```
test/boost/network_topology_strategy_test.cc(0): Entering test case "testCalculateEndpoints"
unknown location(0): fatal error: in "testCalculateEndpoints": std::out_of_range: _Map_base::at
./seastar/src/testing/seastar_test.cc(43): last checkpoint
test/boost/network_topology_strategy_test.cc(0): Leaving test case "testCalculateEndpoints"; testing time: 15192us
test/boost/network_topology_strategy_test.cc(0): Entering test case "test_invalid_dcs"
network_topology_strategy_test: ./seastar/include/seastar/core/future.hh:634: void seastar::future_state<seastar::internal::monostate>::set(A &&...) [T = seastar::internal::monostate, A = <>]: Assertion `_u.st == state::future' failed.
Aborting on shard 0.
Backtrace:
  0x0000000002825e94
  0x000000000282ffa9
  0x00007fd065f971df
  /lib64/libc.so.6+0x000000000003dbc4
  /lib64/libc.so.6+0x00000000000268a3
  /lib64/libc.so.6+0x0000000000026788
  /lib64/libc.so.6+0x0000000000035fc5
  0x0000000000b484cf
  0x0000000002a7c69f
  0x0000000002a7c62f
  0x0000000000b47b9e
  0x0000000002595da2
  0x0000000002595913
  0x0000000002a83a31

```

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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