We should introduce tests verifying various cases of blocked requests

We already have the following tests:

• browser_net_block.js
• browser_net_block-context.js
• browser_net_block-csp.js
• browser_net_block-draganddrop.js
• browser_net_block-pattern.js
• browser_net_cached-status.js

We should introduce new:

• request blocked by CORS (see bug 1157817)
• request blocked due to cancellation (e.g. by an extension) (see bug 1555057)
• request blocked by tracking protection (see bug 1157817)
• mixed content blocks insecure requests

Some comments:

• We should verify the reason
• We should verify the addon-id (if and addon is responsible for blocking)
• We should extend existing tests where appropriate

Honza