Currently, there is no way to opt-out of loading plugins from the default plugin path. Especially for testing, we should add API to only load plugins from the custom plugin path. Note that this should be the actual fix for https://bugs.webkit.org/show_bug.cgi?id=172840.
<rdar://problem/33404303>
We should be able to cherry-pick which plugins we use, that is the point of decidePolicyForPluginLoad (from the WTR::TestController). However, ITP isn't respecting this option.
Plugins are not the issue here, the test suite set-up for ITP is the issue. I am closing this bug, I will link to the fix once I've finished putting it together.
https://bugs.webkit.org/show_bug.cgi?id=174720 tracks the fix in WebKitTestRunner.