$wpdb error suppression should always inherit previous value when being toggled

[wonderboymusic]

There are some places in core that explicitly set $wpdb->suppress_errors to false via the $wpdb->suppress_errors() method. Other places handle this correctly:

$suppress = $wpdb->suppress_errors();
code_that_might_blow_up();
$wpdb->suppress_errors( $suppress );

Perfect example of why this matters: multisite unit tests will explode if database errors are output. Even if $wpdb->suppress_errors() is called in setUp or in individual tests, install_blog() is overriding this value by setting it to false after it itself suppresses the errors.

[wonderboymusic]

In 26252:

For unit tests that call wpmu_create_blog(), Blog factory, or installation code that attempts to clear transients: suppress database errors on setUp and restore on tearDown.

There are a few places in core that were preventing this from working by explicity setting $wpdb->suppress_errors to false. Instead, they should inherit the value that existed before errors were suppressed.

This allows Multisite unit tests to run without explosive database errors, and allows $wpdb->suppress_errors to be overridden all the way down the chain.

Fixes #26102.