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.

module_build_service/__init__.py

@@ -24,7 +24,6 @@ import pkg_resources
 
 from flask import Flask, has_app_context, url_for
 from flask_sqlalchemy import SQLAlchemy
-from sqlalchemy.pool import StaticPool
 
 from module_build_service.common.config import config_section
 from module_build_service.web.proxy import ReverseProxy
@@ -40,27 +39,7 @@ app.wsgi_app = ReverseProxy(app.wsgi_app)
 app.config.from_object(config_section)
 
 
-class MBSSQLAlchemy(SQLAlchemy):
-    """
-    Inherits from SQLAlchemy and if SQLite in-memory database is used,
-    sets the driver options so multiple threads can share the same database.
-
-    This is used *only* during tests to make them faster.
-    """
-
-    def apply_driver_hacks(self, app, info, options):
-        if info.drivername == "sqlite" and info.database in (None, "", ":memory:"):
-            options["poolclass"] = StaticPool
-            options["connect_args"] = {"check_same_thread": False}
-            try:
-                del options["pool_size"]
-            except KeyError:
-                pass
-
-        super(MBSSQLAlchemy, self).apply_driver_hacks(app, info, options)
-
-
-db = MBSSQLAlchemy(app)
+db = SQLAlchemy(app)
 
 
 def get_url_for(*args, **kwargs):