Add tests to assert if subdirectories with special characters are served correctly

[sidharthachatterjee]

What kind of change does this PR introduce?

Adds some tests

Did you add tests for your changes?

Yes

Summary

Gatsby uses webpack-dev-middleware and I wrote these tests while trying to debug gatsbyjs/gatsby#11348 so I figured I might as well contribute back

Please feel free to reject if this edge case is too specific

Does this PR introduce a breaking change?

No

Other information

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #365 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #365   +/-   ##
=======================================
  Coverage   96.83%   96.83%           
=======================================
  Files           7        7           
  Lines         253      253           
=======================================
  Hits          245      245           
  Misses          8        8

---

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[codecov]

Codecov Report

Merging #365 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #365   +/-   ##
=======================================
  Coverage   96.83%   96.83%           
=======================================
  Files           7        7           
  Lines         253      253           
=======================================
  Hits          245      245           
  Misses          8        8

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 71c3e20...c93784f. Read the comment docs.

[alexander-akait]

Thanks for pr