Add unit tests on PHP 7.3

[phil-davis]

Issue https://github.com/owncloud/enterprise/issues/3187

[codecov]

Codecov Report

Merging #226 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #226   +/-   ##
=========================================
  Coverage     84.48%   84.48%           
  Complexity      310      310           
=========================================
  Files            22       22           
  Lines           973      973           
=========================================
  Hits            822      822           
  Misses          151      151

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