Add unit tests on PHP 7.3

[phil-davis]

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

[codecov]

Codecov Report

Merging #450 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #450   +/-   ##
=========================================
  Coverage     35.64%   35.64%           
  Complexity     1340     1340           
=========================================
  Files            31       31           
  Lines          3785     3785           
=========================================
  Hits           1349     1349           
  Misses         2436     2436

---

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[codecov]

Codecov Report

Merging #450 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #450   +/-   ##
=========================================
  Coverage     35.64%   35.64%           
  Complexity     1340     1340           
=========================================
  Files            31       31           
  Lines          3785     3785           
=========================================
  Hits           1349     1349           
  Misses         2436     2436

---

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 ede0781...5f7d217. Read the comment docs.