Intermittent errors with D7 SQLite tests

Problem/Motivation

With #2978575: Mysql 8 Support on Drupal 7 deployed, D7 SQLite tests are now generally working (there are still a few failures which need to be fixed in D7 itself) but we're hitting intermittent errors on some tests, e.g.:

12:48:56 DatabaseSchemaObjectExistsException: Table variable already exists. in /var/www/html/includes/database/schema.inc:705
12:48:56 Stack trace:
12:48:56 #0 /var/www/html/includes/database/database.inc(2803): DatabaseSchema->createTable('variable', Array)
12:48:56 #1 /var/www/html/includes/common.inc(7191): db_create_table('variable', Array)
12:48:56 #2 /var/www/html/modules/system/system.install(600): drupal_install_schema('system')
12:48:56 #3 /var/www/html/includes/module.inc(934): system_install()

...where a retest ran through okay.

I suspect this may be down to concurrency, where drupalci is typically running with --concurrency "32".

Steps to reproduce

Example patch which hit an exception once, and worked okay on retest:

https://www.drupal.org/project/drupal/issues/3172878#comment-13842745

Proposed resolution

As SQLite tests on D7 are very fast on drupalci, can we try running with a lower concurrency to see if SQLite copes with that better?

Remaining tasks

Review patch etc..

User interface changes

n/a

API changes

n/a

Data model changes

n/a