tests: test_secondary_mode_eviction: avoid use of mocked statvfs (#6698)

## Problem

Test sometimes fails with `used_blocks > total_blocks`, because when
using mocked statvfs with the total blocks set to the size of data on
disk before starting, we are implicitly asserting that nothing at all
can be written to disk between startup and calling statvfs.

Related: https://github.com/neondatabase/neon/issues/6511

## Summary of changes

- Use HTTP API to invoke disk usage eviction instead of mocked statvfs

test_runner/regress/test_disk_usage_eviction.py

@@ -893,37 +893,14 @@ def test_secondary_mode_eviction(eviction_env_ha: EvictionEnv):
         # in its heatmap
         ps_secondary.http_client().tenant_secondary_download(tenant_id)
 
-    # Configure the secondary pageserver to have a phony small disk size
-    ps_secondary.stop()
     total_size, _, _ = env.timelines_du(ps_secondary)
-    blocksize = 512
-    total_blocks = (total_size + (blocksize - 1)) // blocksize
+    evict_bytes = total_size // 3
 
-    min_avail_bytes = total_size // 3
-
-    env.pageserver_start_with_disk_usage_eviction(
-        ps_secondary,
-        period="1s",
-        max_usage_pct=100,
-        min_avail_bytes=min_avail_bytes,
-        mock_behavior={
-            "type": "Success",
-            "blocksize": blocksize,
-            "total_blocks": total_blocks,
-            # Only count layer files towards used bytes in the mock_statvfs.
-            # This avoids accounting for metadata files & tenant conf in the tests.
-            "name_filter": ".*__.*",
-        },
-        eviction_order=EvictionOrder.ABSOLUTE_ORDER,
-    )
-
-    def relieved_log_message():
-        assert ps_secondary.log_contains(".*disk usage pressure relieved")
-
-    wait_until(10, 1, relieved_log_message)
+    response = ps_secondary.http_client().disk_usage_eviction_run({"evict_bytes": evict_bytes})
+    log.info(f"{response}")
 
     post_eviction_total_size, _, _ = env.timelines_du(ps_secondary)
 
     assert (
-        total_size - post_eviction_total_size >= min_avail_bytes
-    ), "we requested at least min_avail_bytes worth of free space"
+        total_size - post_eviction_total_size >= evict_bytes
+    ), "we requested at least evict_bytes worth of free space"