Fix skipping of ConnectionUnitTest for non-MySQL dbs

Problem/Motivation

Looks like ConnectionUnitTest doesn't actually get skipped because of a very small logic snafu:

https://git.drupalcode.org/project/drupal/-/blob/7.73/modules/simpletest...

    $this->skipTest = (bool) $connection_info['default']['driver'] != 'mysql';

Steps to reproduce

php > var_dump((bool) 'sqlite' != 'mysql');

bool(false)

php > var_dump((bool) ('sqlite' != 'mysql'));

bool(true)

Proposed resolution

Fix the logic so that the test is skipped for non-MySQL drivers.

Remaining tasks

Follow up if we want to fix PostgreSQL tests too at some point - backport of: #3128880: Make ConnectionUnitTest also run for PostgreSQL

...for now though, I'd expect this patch to fix (by skipping) a test which will fail in PostgreSQL.

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet

n/a