Support code coverage in javascript test

After fixing #791 , we should make the tests support code coverage to make the tests more useful.

If the web test is run through chromedp, then one option is to make use of the chrome devtools profiler ([profiler.startPreciseCoverage](https://chromedevtools.github.io/devtools-protocol/tot/Profiler/#method-startPreciseCoverage) method) to collect the coverage info of the javascript files, and convert it into the code coverage format (example format from the current report coverage.txt, generated by [go -coverprofile in test.sh](https://github.com/google/pprof/blob/main/test.sh): `github.com/google/pprof/profile/prune.go:152.2,152.12 1 2`)