Allow headless nightwatch tests

[alex-mo]

redirects_app: run tests with HEADLESS=1 yarn e2e for headless browser

[codecov]

Codecov Report

Merging #134 into 7.x-1.x will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             7.x-1.x    #134   +/-   ##
=========================================
  Coverage       35.4%   35.4%           
  Complexity      1674    1674           
=========================================
  Files            228     228           
  Lines           8626    8626           
=========================================
  Hits            3054    3054           
  Misses          5572    5572

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[torotil]

Tests fail for me in this branch if I invoke them as described.

[alex-mo]

how do they fail? (because for me the tests are all green. I only had issues getting selenium to work, but the HEADLESS env should be fixing this...)