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.python.cpython_2_7_15

@@ -4,4 +4,5 @@ ADD . .
 RUN cp flatc_debian_stretch flatc
 WORKDIR /code/tests
 RUN python --version
+RUN pip install coverage
 RUN ./PythonTest.sh