Fix location for get-pip.py used in tests. #8925

In #7915 we factored out a helper class for managing virtual envs during testing (see Venvs in src/test/interpreters/locators/workspaceVirtualEnvService.test.ts). As part of that, we addressed an issue with creating a "venv" on Ubuntu using a system python. That included downloading get-pip.py as necessary.

Unfortunately we downloaded that file to a location inside the projects source tree, so git status shows it (as an unmanaged file). This is mildly problematic as we don't want to accidentally commit the file into the repo. In #8902 we added an entry to .gitignore for src/test/get-pip.py, as a temporary solution. We need a permanent solution.

Options:

1. use an existing vendored copy at ./data/get-pip.py (added in #Fix problem with other kernels that use python being picked. #5628)
2. vendor a second copy of get-pip.py into the repo (in a better location)
3. download get-pip.py into a directory in the repo that is ignored in .gitignore
4. download get-pip.py into a non-temp directory outside the repo
5. download get-pip.py into temp directory (outside the repo)

Tasks:

• settle on a permanent solution
• implement it
• remove that .gitignore entry