Ensure that the latest version of `micropip` is used for testing

[agriyakhetarpal]

use the fixture selenium_standalone_micropip, not selenium. Otherwise the micropip in the Pyodide distribution will be used (which was 0.8.0 in 0.27.2 and 0.6.0 in 0.27.0a2). I think we should find all invalid fixtures and replace it.

Originally posted by @ryanking13 in #183 (comment)

[ryanking13]

Thanks for opening the issue Agriya! I often forget about the comments that I wrote, so it is very helpful to make them separate issues.