Manual cleanup and organization of db_tests w/new gap coverage.

Clean up some of the assertions, merge some tests.

peewee.py

@@ -3562,7 +3562,7 @@ class Database(_callable_context_manager):
         return _BoundModelsContext(models, self, bind_refs, bind_backrefs)
 
     def get_noop_select(self, ctx):
-        return ctx.sql(Select().columns(SQL('0')).where(SQL('0')))
+        return ctx.literal('SELECT 0 WHERE 0')
 
     @property
     def Model(self):
@@ -4335,7 +4335,7 @@ class PostgresqlDatabase(Database):
         return fn.to_timestamp(date_field)
 
     def get_noop_select(self, ctx):
-        return ctx.sql(Select().columns(SQL('0')).where(SQL('false')))
+        return ctx.literal('SELECT 0 WHERE false')
 
     def set_time_zone(self, timezone):
         self.execute_sql('set time zone \'%s\';' % timezone.replace("'", "''"))