In preparation for the session initialization rewrite using the plugins JSON data...

Despite there being the session pytest fixture for initializing a Streamlink session object without loading any plugins, lots of tests are currently still manually initializing the session, which leads to all plugins being loaded every time. This slows down the tests significantly. On my local system, these changes improve the run time from ~15s to ~10s.

The upcoming plugins JSON data implementation will make lots of changes to the Streamlink class, so patching the load_builtin_plugins() method won't be necessary anymore in the tests afterwards.