rptest: reduce cache eviction throttling for space leak test

This test is too slow with default configuration making the test flaky.
Instead of raising the timeouts I'm trying to reduce the cache eviction
throttling which makes the test 3x faster.

The test became flaky after in-memory trim was introduced in
redpanda-data#21556.

The main insight was provided by https://github.com/abhijat in a private
exchange:

> I think it might be the extra throttling. With the carry over
> disabled, we always have to do a trim when reserving space, which
> results in a lot more throttling and sleep:
>
> ```
> $ grep -Ri "Cache trimming throttled" * | grep -c cache
> 139
> ```
>
> With the carryover list in place, about half of the calls to reserve
> space end up in an early return because the list provides enough room
> to clear up space, which does not cause the trimming to be throttled
> as much:
>
> ```
> $ grep -Ri "Cache trimming throttled" * | grep -c cache
> 63
> ```
>
> Although that doesn't explain how this test used to work before, IIRC
> carryover is a fairly new feature

Fixes redpanda-data#21597

tests/rptest/tests/test_si_cache_space_leak.py

@@ -59,6 +59,7 @@ def __init__(self, test_context, *args, **kwargs):
             'segment_fallocation_step': 0x1000,
             'retention_local_target_bytes_default': self._segment_size,
             'retention_bytes': self._segment_size * 5,
+            'cloud_storage_cache_check_interval': 500,
         }
         super().__init__(test_context,
                          num_brokers=3,