Commit instead of flushing in unit test

Rationale:
We use an in-memory sqlite database for quota tests. With SA 2.0 (or
with the `future` flag enabled on the engine), the following conflict
happens: (this is a simplified model)

foo = Foo()
session.add(foo)
session.flush()

engine = session.get_bind()
with engine.connect() as conn:
    conn.execute(some-sql)

foo.bar = "new value"
session.commit() # BOOM!!!!  sqlalchemy.orm.exc.StaleDataError: UPDATE statement on table 'galaxy_user' expected to update 1 row(s); 0 were matched.

Reason for BOOM:
With an in-memory database, the underlying dbapi_connection object is
the same for the session and the engine.connect(). Here's what happens:

line 10: foo is flushed to the db tmp buffer
line 14: conn is closed on exit from context manager, which issues a
rollback, which rolls back whatever is in the tmp buffer - so foo is
never inserted.
line 16: foo is updated
line 17: error happens: the session thinks it's updating foo's record in
the db, but that record does not exist therefore, "0 rows matched".

Solution: commit instead of flushing - then foo is inserted.

test/unit/data/test_galaxy_mapping.py

@@ -56,12 +56,12 @@ def setUpClass(cls):
     @classmethod
     def persist(cls, *args, **kwargs):
         session = cls.session()
-        flush = kwargs.get("flush", True)
+        commit = kwargs.get("commit", True)
         for arg in args:
             session.add(arg)
-            if flush:
-                session.flush()
-        if kwargs.get("expunge", not flush):
+            if commit:
+                session.commit()
+        if kwargs.get("expunge", not commit):
             cls.expunge()
         return arg  # Return last or only arg.
 
@@ -255,7 +255,7 @@ def test_collection_get_interface(self):
             model.DatasetCollectionElement(collection=c1, element=d1, element_identifier=f"{i}", element_index=i)
             for i in range(elements)
         ]
-        self.persist(u, h1, d1, c1, *dces, flush=False, expunge=False)
+        self.persist(u, h1, d1, c1, *dces, commit=False, expunge=False)
         self.model.session.flush()
         for i in range(elements):
             assert c1[i] == dces[i]