Update test-isfile to correct response for "/"

[Patochun]

I Think the good answer for "/" is a directory, not a file.

PR Summary

I recommand to change EXPECT_TRUE by EXPECT_FALSE for the case test "/".

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.
• User-facing changes
  • Not Applicable
  • OR
  • User-facing Documentation needed
    • Issue filed - Issue link:
• Testing - New and feature
  • Not Applicable or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
    • Add [feature] if the change is significant or affects feature tests

[iSazonov]

@daxian-dbw @TravisEz13 Do you understand the PR?

[TravisEz13]

@dantraMSFT Can you review?

[Patochun]

The change does not concern the error provided by Travis

[iSazonov]

I restarted CI Travis Linux - Update-Help test temporary failed.