Use the new 18.04 for the ones where the build succeeded and the
previous 16.04 where it didn't.

Some tests are not run when OPcache is not available.
This should make the results more accurate.

* Add an Azure Publish Code Coverage Results task
* Add `make gcovr-html` to generate a gcovr test coverage report in HTML
* Add `make gcovr-xml` to generate a gcovr test coverage report in XML
* Remove `test` target dependency from `make lcov-html`;
  Run the two targets together instead: `make test lcov-html`.

Re: https://github.com/php/php-src/pull/4739#issuecomment-534911441

See: https://externals.io/message/107113,
https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/test/publish-code-coverage-results?view=azure-devops,
and https://github.com/php/php-src/pull/4759.

Try to do this using "variables" -- apparently these get uppercased
and are made available as environment variables.

Perform PHPUnit installation under php7.3 -- which will also make
it work on master.

Also properly resolve the tty issue by specifying excluded groups
during the test run.

Run some open-source projects through an aggressive debug
configuration with asan and ubsan. We don't care about test results,
only check that we don't assert or crash.

Currently testing laravel, symfony and amp.

It is already installed on azure, and this installation step is
very unreliable lately.

Closes GH-4395