Go back to using a file-backed SQLite database for tests

Using a memory database causes tests/test_build to intermittently
fail, because using the same pysqlite3 connection object from multiple
threads - as was done so that the threads shared the same memory database
- is not, in the end, thread safe. One thread will stomp on the transaction
state of other threads, resulting in errors from starting a new transaction
when another is already in progress, or trying to commit a transaction
that is not in progress.

To avoid a significant speed penalty, the session-scope fixture sets up
a database in the pytest temporary directory, which will typically be on
tmpfs. Time to complete all tests:

 memory backend:               38 seconds
 file on tmpfs:                40 seconds
 file on nvme ssd with btrfs: 137 seconds

MBSSQLAlchemy, which attempted to make the memory backend work, is removed.

Session hooks are installed on the Session class rather than on the
scoped_session instance - this works better when we're changing from
one database to another at test setup time.

tests/conftest.py

@@ -281,6 +281,17 @@ def provide_test_client(request):
 
 
 @pytest.fixture(autouse=True, scope="session")
-def create_database(request):
+def create_database(request, tmpdir_factory):
     """Drop and recreate all tables"""
-    clean_database(add_platform_module=False, add_default_arches=False)
+
+    # pysqlite + the SQLite memory backend doesn't handle multiple threads usefully:
+    # either each thread gets its own backend, or the threads stomp on each others
+    # transaction state. So, before running tests, replace the memory backend with
+    # a file backend - if this ends up on tmpfs, its almost as fast as the memory
+    # backend.
+    database_uri = None
+    if module_build_service.app.config["SQLALCHEMY_DATABASE_URI"] == 'sqlite:///:memory:':
+        database_path = str(tmpdir_factory.getbasetemp() / "test.db")
+        database_uri = 'sqlite:///' + database_path
+
+    clean_database(database_uri=database_uri, add_platform_module=False, add_default_arches=False)