Preserve all query params expect `test` and `grep` for grep links in HTML reporter

There are webapps that change behavior depending on URL query parameter values. For example, this demo application chooses its engine based on the value of the `engine` query parameter:

http://tkareine.org/hackjkl-intro-frp/formapp/index.html?test&engine=bacon

Without this change, when we click on a specific test context, we lose engine selection:

http://tkareine.org/hackjkl-intro-frp/formapp/index.html?grep=After%20loading%20application%20when%20entering%20space%20for%20search

This change preserves all query parameters except `test` and `grep` when creating grep links in HTML reporter.

If needed, location hash can be easily preserved, too. See GH-827.