Speed up SQLite unit test CI (#15334)

Tests now take 40% of the time.

tests/server.py

@@ -16,6 +16,7 @@ import json
 import logging
 import os
 import os.path
+import sqlite3
 import time
 import uuid
 import warnings
@@ -79,7 +80,9 @@ from synapse.http.site import SynapseRequest
 from synapse.logging.context import ContextResourceUsage
 from synapse.server import HomeServer
 from synapse.storage import DataStore
+from synapse.storage.database import LoggingDatabaseConnection
 from synapse.storage.engines import PostgresEngine, create_engine
+from synapse.storage.prepare_database import prepare_database
 from synapse.types import ISynapseReactor, JsonDict
 from synapse.util import Clock
 
@@ -104,6 +107,10 @@ P = ParamSpec("P")
 # the type of thing that can be passed into `make_request` in the headers list
 CustomHeaderType = Tuple[Union[str, bytes], Union[str, bytes]]
 
+# A pre-prepared SQLite DB that is used as a template when creating new SQLite
+# DB each test run. This dramatically speeds up test set up when using SQLite.
+PREPPED_SQLITE_DB_CONN: Optional[LoggingDatabaseConnection] = None
+
 
 class TimedOutException(Exception):
     """
@@ -899,6 +906,22 @@ def setup_test_homeserver(
             "args": {"database": test_db_location, "cp_min": 1, "cp_max": 1},
         }
 
+        # Check if we have set up a DB that we can use as a template.
+        global PREPPED_SQLITE_DB_CONN
+        if PREPPED_SQLITE_DB_CONN is None:
+            temp_engine = create_engine(database_config)
+            PREPPED_SQLITE_DB_CONN = LoggingDatabaseConnection(
+                sqlite3.connect(":memory:"), temp_engine, "PREPPED_CONN"
+            )
+
+            database = DatabaseConnectionConfig("master", database_config)
+            config.database.databases = [database]
+            prepare_database(
+                PREPPED_SQLITE_DB_CONN, create_engine(database_config), config
+            )
+
+        database_config["_TEST_PREPPED_CONN"] = PREPPED_SQLITE_DB_CONN
+
     if "db_txn_limit" in kwargs:
         database_config["txn_limit"] = kwargs["db_txn_limit"]