Errors in performance tests with inaccessible external CSS/JS assets

Problem/Motivation

While implementing a performance test for a project, using PerformanceTrait, I'm hitting this error:

TypeError: strlen(): Argument #1 ($string) must be of type string, false given/var/www/html/web/core/tests/Drupal/Tests/PerformanceTestTrait.php:406

The problem is that my site loads external CSS/JS which have specific access policies making them unavailable in the testing environment (403). It's not in the scope here to debate why they are doing this, this could happen for various reasons.

Steps to reproduce

From the page requests an external CSS asset that return 403.

Proposed resolution

EITHER,

Check if a valid string has been returned when GETing the CSS/JS assets and ignore that asset if not,

OR,

Allow a list of regex patterns of URLs to be bypassed in \Drupal\Tests\PerformanceTestTrait::collectNetworkData().

Probably, the later is more solid because it allows to have the same baseline every test run while the first option might have temporary random behavior.

Remaining tasks

Decide the way to go. EDIT: Discusses with @catch to go with the 2nd option.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.