set the version for integration tests using a command line flag

[marten-seemann]

We've had a lot of loops over the supported versions in our self tests. This is annoying for multiple reasons:

1. We always have to fill out the quic.Config to make sure that these versions are actually used.
2. It creates a lot of indentation of the test code.
3. Running the suite takes a long time, since all versions are tested. Most changes we make are not version-dependent though.

By setting the version as a command line parameter, we can still test all versions (mostly on CI), but just run using v1 locally.

This requires splitting out the version negotiation tests into a separate package.

[codecov]

Codecov Report

Merging #3782 (2b0a03a) into master (172123c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3782   +/-   ##
=======================================
  Coverage   84.24%   84.24%           
=======================================
  Files         142      142           
  Lines       14089    14090    +1     
=======================================
+ Hits        11868    11869    +1     
  Misses       1801     1801           
  Partials      420      420           

Impacted Files	Coverage Δ
http3/client.go	85.48% <100.00%> (+0.05%)	⬆️