Reactivate Mempurge feature in crash test. (#9684)

Summary:
Set `experimental_mempurge_threshold` back to `lambda: 10.0*random.random()` in crash test, reverting https://github.com/facebook/rocksdb/issues/8958 after fix provided in https://github.com/facebook/rocksdb/issues/9671 .

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9684

Reviewed By: pdillinger

Differential Revision: D34820257

Pulled By: bjlemaire

fbshipit-source-id: 1e5ae8c872c4ac4c4267c990ac5e3e793d77908c

tools/db_crashtest.py

@@ -237,10 +237,7 @@ whitebox_default_params = {
 simple_default_params = {
     "allow_concurrent_memtable_write": lambda: random.randint(0, 1),
     "column_families": 1,
-    # TODO: re-enable once below loop succeeds for a while (a few minutes should
-    # suffice):
-    # `while rm -rf /dev/shm/single_stress && ./db_stress --clear_column_family_one_in=0 --column_families=1 --db=/dev/shm/single_stress --experimental_mempurge_threshold=5.493146827397074 --flush_one_in=10000 --reopen=0 --write_buffer_size=262144 --value_size_mult=33 --max_write_buffer_number=3 -ops_per_thread=10000; do : ; done`
-    "experimental_mempurge_threshold": 0,
+    "experimental_mempurge_threshold": lambda: 10.0*random.random(),
     "max_background_compactions": 1,
     "max_bytes_for_level_base": 67108864,
     "memtablerep": "skip_list",