Remove usage of badssl.com in k6 tests

[mstoykov]

Currently, we are making external requests to badssl.com to test some specific SSL errors and compatibility with old versions 1 2 3.

This probably need to be looked over and seen if they are still all needed, there seems to be some repetition at first glance, and tried to use a local HTTPS server, which should be possible for all of them.

There is also a sample test which probably can stay as it is, I am just mentioning it for completeness.

The idea with not using an external site is both to make the tests more resilient to outside change, but also to make them faster, as making network requests outside of localhost is definitely slower. I would also recommend on not just adding all of this functionality to the httpmultibin but instead just specific server or even endpoints (not certain if possible) for the cases we care about.

This was extracted as the last part of #537, before closing it.