Add E2E test for new Attributes Filters in Triggers

[nachocano]

Problem
In 0.8 we included an attributes filter for Triggers to be able to filter on any CloudEvents attributes. We should have an E2E test for it.

Persona:
Which persona is this feature for?
Developers

Exit Criteria
Some E2E tests showing that the attribute-based filtering is working.

Time Estimate (optional):
1 day

Additional context (optional)
Note this known issue

/kind good-first-issue

[nachocano]

fyi @Fredy-Z

[capri-xiyue]

/assign @capri-xiyue