Test: `captures stylesheet rules` not deterministic

[Juice10]

The captures stylesheet rules test fails randomly sometimes without any related code being changed.

Example of the same code running twice and having two different outcomes in Travis CI:

• Build Failed: 4ab4883
• Build Passed: e72e201

Another example of code being modified that doesn't have to do with this test but the test is randomly failing and passing:
#294

[Yuyz0112]

Yes, there are some fragile test cases, most of them are causing by the network(like load async stylesheet).

Do not have time to investigate in every case at this moment, but definitely something we can do to improve the dev process.