github-actions: adapt cypress test to new server code

[da-h]

Description

This PR adapts the cypress test workflow to the new server code (for future reference: #858). Note that it removes the additional checks introduced in #858 (commit 1a34bb6b) to have the test work across the different versions for the server (before and after refactoring).

(The test failed here because of a missing argument for test. It should have been if test -f "py/visdom/server.py" instead of if test "py/visdom/server.py". Nevertheless, the extra test-case here is not required anymore.)

How Has This Been Tested?

Tested on my own fork, see here

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactor or cleanup (changes to existing code for improved readability or performance)

Checklist:

• [ ] I adapted the version number under py/visdom/VERSION according to Semantic Versioning
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[da-h]

Added a commit to let all tests use the visdom command instead of the "local" call python3 py/visdom/server. (The ./github/actions/prepare/action.yml installs the packages anyways before any test.)

• The former is what users actually would call.
  The benefit is that using this version for actual testing would automatically detect errors like in #878.
• The latter is a bit problematic, because the new file structure uses absolute imports of the setup.py installed package visdom. I.e. running python3 py/visdom/server will not work if the code has not been installed.
  Note that this would not have been a problem for a "single-file" script.

[JackUrb]

Fair point - the single file script was able to avoid install, but too unwieldy to continue to maintain. I think the tradeoff is worthwhile.