Add a test for ls for a single file

[freitagbr]

Fixes #732

Adds a single test to test/ls.js that runs ls on a single file, which is not covered by the existing tests.

[codecov-io]

Codecov Report

Merging #784 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #784   +/-   ##
=======================================
  Coverage   95.39%   95.39%           
=======================================
  Files          33       33           
  Lines        1239     1239           
=======================================
  Hits         1182     1182           
  Misses         57       57

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[nfischer]

Doesn't change coverage? Maybe the lines were covered by some other test?

[freitagbr]

All of the code paths were probably covered by the other tests, but not in the combination that the new test does. It's good having this test around because it tests a common use case for ls.