Do not assume platform will always be the same when downloading VSCode

[J-Fields]

I work on a Windows machine but most of the time run my extension tests in a docker container running linux. If I do this, then run my tests from Windows directly, it hits a ENOENT because it sees that the VSCode version folder (.vscode-test/vscode-1.51.1, for instance) exists and assumes the .exe exists - but only the linux binary does. The reverse happens if I run the test suite from Windows, then from the linux image.

Example output:

$ yarn test
yarn run v1.22.5
warning vim@1.18.4: The engine "vscode" appears to be invalid.
$ node ./out/test/runTest.js
Found .vscode-test/vscode-1.51.1. Skipping download.
Test error: Error: spawn C:\Users\jfields\Desktop\Vim\.vscode-test\vscode-1.51.1\Code.exe ENOENT
Exit code:   -4058
Done

Failed
Failed to run tests
error Command failed with exit code 1.

I think this line is the culprit - it should look one level deeper to see if the requested platform's binary exists (not any binary whatsoever).