Fix flaky tracing test that fails with web_vital span err

[inancgumus]

Brief summary

When the integration tests are ran in CI, the workflow often fails on one of the TestTracing, specifically the web_vital sub test.

When this test fails we can usually rerun the failed workflow and the test will pass the second time round, so it's flakey. It would be good if this wasn't the case so that it gives us confidence in our tests and workflows, as well as making it easier to contribute to for external contributors.

xk6-browser version

fcc7f9e

OS

NA

Chrome version

NA

Docker version and image (if applicable)

NA

Steps to reproduce the problem

Run the test in CI, e.g.: https://github.com/grafana/xk6-browser/actions/runs/8138217177/job/22238484745?pr=1228

Expected behaviour

The integration test passes.

Actual behaviour

It fails with:

--- FAIL: TestTracing (3.71s)
    tracing_test.go:161: 
        	Error Trace:	/home/runner/work/xk6-browser/xk6-browser/tests/tracing_test.go:161
        	Error:      	Received unexpected error:
        	            	"web_vital" span was not found
        	Test:       	TestTracing

### Tasks
- [ ] https://github.com/grafana/xk6-browser/pull/1262
- [ ] https://github.com/grafana/k6/pull/3709

[inancgumus]

It happens every other day. Here's the latest one:

time="2024-03-05T06:01:29Z" level=error msg="error on evaluating window.k6SpanId: evaluating JS in global context: context canceled" category="FrameSession:onFrameNavigated" elapsed="0 ms" source=browser
time="2024-03-05T06:01:29Z" level=error msg="Failed to load resource: the server responded with a status of 404 (Not Found)" browser_source=network line_number=0 source=browser stacktrace="<nil>" url="http://127.0.0.1:35093/static/empty.html"
time="2024-03-05T06:01:29Z" level=error msg="error on evaluating window.k6SpanId: evaluating JS in global context: context canceled" category="FrameSession:onFrameNavigated" elapsed="0 ms" source=browser
--- FAIL: TestTracing (6.88s)
    tracing_test.go:1[61](https://github.com/grafana/xk6-browser/actions/runs/8151736116/job/22280064609#step:4:62): 
        	Error Trace:	/home/runner/work/xk6-browser/xk6-browser/tests/tracing_test.go:161
        	Error:      	Received unexpected error:
        	            	"web_vital" span was not found
        	Test:       	TestTracing

Interesting points are:

"Failed to load resource: the server responded with a status of 404 (Not Found)"
->
FrameSession:onFrameNavigated
->
"error on evaluating window.k6SpanId
->
"evaluating JS in global context: context canceled"