Updates file unit tests to be Windows compatible

[tylersmalley]

Summary

Replaces #23674
Related to #21315

Resolves the following mocha test failing on Windows:

Failures:

1. dev/File #getRelativePath() returns the path relative to the repo root:

      Error: expected 'src\\dev\\__tests__\\file.js' to sort of equal 'src/dev/__tests__/file.js'
      + expected - actual

      -src\dev\__tests__\file.js
      +src/dev/__tests__/file.js
      
      at Assertion.assert (node_modules\expect.js\index.js:96:13)
      at Assertion.eql (node_modules\expect.js\index.js:230:10)
      at Context.it (C:/Users/vagrant/kibana/src/dev/__tests__/file.js:41:41)

2. dev/File #getRelativeParentDirs() returns the parents of a file, stopping at the repo root, in descending order:

      Error: expected [ 'src\\dev\\__tests__', 'src\\dev', 'src' ] to sort of equal [ 'src/dev/__tests__', 'src/dev', 'src' ]
      + expected - actual

       [
      -  "src\\dev\\__tests__"
      -  "src\\dev"
      +  "src/dev/__tests__"
      +  "src/dev"
         "src"
       ]
      
      at Assertion.assert (node_modules\expect.js\index.js:96:13)
      at Assertion.eql (node_modules\expect.js\index.js:230:10)
      at Context.it (C:/Users/vagrant/kibana/src/dev/__tests__/file.js:71:47)

3. src/dev/npm/installed_packages getInstalledPackages() reads all installed packages of a module:

 Error: expected [ { name: 'dep1',
    version: '0.0.2',
    repository: 'https://github.com/mycorp/dep1',
    licenses: [ 'Apache-2.0' ],
    directory: 'C:\\Users\\vagrant\\kibana\\src\\dev\\npm\\__tests__\\fixtures\\fixture1\\node_modules\\dep1',
    relative: 'node_modules\\dep1' },
  { name: 'privatedep',
    version: '0.0.2',
    repository: 'https://github.com/mycorp/privatedep',
    licenses: [ 'Apache-2.0' ],
    directory: 'C:\\Users\\vagrant\\kibana\\src\\dev\\npm\\__tests__\\fixtures\\fixture1\\node_modules\\privatedep',
    relative: 'node_modules\\privatedep' } ] to sort of equal [ { name: 'dep1',
    version: '0.0.2',
    licenses: [ 'Apache-2.0' ],
    repository: 'https://github.com/mycorp/dep1',
    directory: 'C:\\Users\\vagrant\\kibana\\src\\dev\\npm\\__tests__\\fixtures\\fixture1\\node_modules\\dep1',
    relative: 'node_modules/dep1' },
  { name: 'privatedep',
    version: '0.0.2',
    repository: 'https://github.com/mycorp/privatedep',
    licenses: [ 'Apache-2.0' ],
    directory: 'C:\\Users\\vagrant\\kibana\\src\\dev\\npm\\__tests__\\fixtures\\fixture1\\node_modules\\privatedep',
    relative: 'node_modules/privatedep' } ]
      + expected - actual

           "licenses": [
             "Apache-2.0"
           ]
           "name": "dep1"
      -    "relative": "node_modules\\dep1"
      +    "relative": "node_modules/dep1"
           "repository": "https://github.com/mycorp/dep1"
           "version": "0.0.2"
         }
         {
           "licenses": [
             "Apache-2.0"
           ]
           "name": "privatedep"
      -    "relative": "node_modules\\privatedep"
      +    "relative": "node_modules/privatedep"
           "repository": "https://github.com/mycorp/privatedep"
           "version": "0.0.2"
         }
       ]
      
      at Assertion.assert (node_modules\expect.js\index.js:96:13)
      at Assertion.eql (node_modules\expect.js\index.js:230:10)
      at Context.it (C:/Users/vagrant/kibana/src/dev/npm/__tests__/installed_packages.js:57:35)

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• [ ] Unit or functional tests were updated or added to match the most common scenarios
• [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

• [ ] This was checked for breaking API changes and was labeled appropriately
• [ ] This includes a feature addition or change that requires a release note and was labeled appropriately

[mistic]

LGTM

[elasticmachine]

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[tylersmalley]

retest

[elasticmachine]

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