Docker tests for Zend PHP and Mono C#; Python coverage; TODO for HHVM PHP (#5125)

* Docker test for Zend PHP 7.3

* hhvm

* move failing hhvm test to TODO zone

* c# mono 5.18 test

* python coverage reports

tests/docker/languages/Dockerfile.testing.php.zend_7_3

@@ -0,0 +1,8 @@
+FROM php:7.3-cli-stretch as base
+WORKDIR /code
+ADD . .
+RUN cp flatc_debian_stretch flatc
+WORKDIR /code/tests
+RUN php --version
+RUN php phpTest.php
+RUN sh phpUnionVectorTest.sh