Functional test server errors out on Windows when running on ES snapshot

[marius-dr]

Kibana version: master

Server OS version: Windows 10

Describe the bug:
Running the test server from Windows will give the following error:

 info  Installing from snapshot
   │ info  version: 7.0.0-alpha1
   │ info  install path: C:\Users\ratonbox\Documents\GitHub\kibana\.es\test-ambq
glgc1wn
   │ info  license: trial
   │ info  downloading from https://snapshots.elastic.co/downloads/elasticsearch
/elasticsearch-7.0.0-alpha1-SNAPSHOT.tar.gz
   │ info  extracting C:\Users\ratonbox\Documents\GitHub\kibana\.es\cache\elasti
csearch-7.0.0-alpha1-SNAPSHOT.tar.gz
   │ info  extracted to C:\Users\ratonbox\Documents\GitHub\kibana\.es\test-ambqg
lgc1wn
   │ info  setting bootstrap password to iamsuperuser
Error: spawn ./bin/elasticsearch-keystore.bat ENOENT

Tracking it down, it's because it downloads the .tar.gz package regardless of the OS. Looks like it comes from here:
https://github.com/elastic/kibana/blob/master/packages/kbn-es/src/install/snapshot.js
in this function:

function getFilename(license, version) {
  const basename = `elasticsearch${
    license === 'oss' ? '-oss-' : '-'
  }${version}`;

  return `${basename}-SNAPSHOT.tar.gz`;
}

ping @tylersmalley

I did try something like this, which fixes the download, but then there are more problems further down the line with the unzip (as it uses the node lib to extract tarballs by default)

const os = require('os');
...
...
...
function getFilename(license, version) {
  var isWin = (os.platform() === 'win32')
  const basename = `elasticsearch${
    license === 'oss' ? '-oss-' : '-'
  }${version}`;
  if (isWin) {
	return `${basename}-SNAPSHOT.zip`
  } else {
  return `${basename}-SNAPSHOT.tar.gz`;}
}

[tylersmalley]

Closing, duplicate of #20063