test/store_test: fix deferred writing test cases

When bitmap allocator is in use different space fragmentation and
allocation strategy cause blob fragmentation. Which in turn impacts the
decision on deferred write applicability.

Fixing by using standalone DB volume to reduce main device space fragmentation.

Fixes: https://tracker.ceph.com/issues/63795
Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>

src/test/objectstore/store_test.cc

@@ -8293,6 +8293,8 @@ TEST_P(StoreTestSpecificAUSize, DeferredOnBigOverwrite2) {
   }
 
   size_t block_size = 4096;
+  SetVal(g_conf(), "bluestore_block_db_create", "true");
+  SetVal(g_conf(), "bluestore_block_db_size", stringify(1 << 30).c_str());
   StartDeferred(block_size);
   SetVal(g_conf(), "bluestore_max_blob_size", "65536");
   SetVal(g_conf(), "bluestore_prefer_deferred_size", "65536");
@@ -8369,6 +8371,8 @@ TEST_P(StoreTestSpecificAUSize, DeferredOnBigOverwrite3) {
   }
 
   size_t block_size = 4096;
+  SetVal(g_conf(), "bluestore_block_db_create", "true");
+  SetVal(g_conf(), "bluestore_block_db_size", stringify(1 << 30).c_str());
   StartDeferred(block_size);
   SetVal(g_conf(), "bluestore_max_blob_size", "65536");
   SetVal(g_conf(), "bluestore_prefer_deferred_size", "65536");
@@ -8447,6 +8451,10 @@ TEST_P(StoreTestSpecificAUSize, DeferredDifferentChunks) {
   size_t alloc_size = 4096;
   size_t large_object_size = 1 * 1024 * 1024;
   size_t prefer_deferred_size = 65536;
+
+  SetVal(g_conf(), "bluestore_block_db_create", "true");
+  SetVal(g_conf(), "bluestore_block_db_size", stringify(1 << 30).c_str());
+
   StartDeferred(alloc_size);
   SetVal(g_conf(), "bluestore_max_blob_size", "131072");
   SetVal(g_conf(), "bluestore_prefer_deferred_size",