Add test coverage report

[nima-karimi]

Add a test coverage report for unit tests and include a check for Travis.

Some useful links:

• https://phpunit.readthedocs.io/en/7.5/code-coverage-analysis.html
• https://tomasvotruba.com/blog/2019/09/02/how-to-speedup-test-coverage-on-travis-by-95-percent/
• https://dev.to/acho_arnold/php-test-coverage-with-travis-ci-and-code-climate-using-clover-eih
• https://docs.travis-ci.com/user/coveralls/

[tomalec]

For the front-end tests/jest we can start with just --coverage, but I 👍 Coveralls

[mikkamp]

After updating to phpunit 9.5 in PR #1867, as long as we have a PHP version with xdebug installed then a coverage report can be produced as follows.

1. Add the following section to phpunit.xml.dist

	<coverage>
		<include>
			<directory suffix=".php">./src</directory>
			<directory suffix=".php">./views</directory>
			<file>google-listings-and-ads.php</file>
		</include>
	</coverage>

2. Setup tests: bin/install-wp-tests.sh <db_name> <db_user> <db_pass>
3. Run the tests with a coverage report: XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-text

Example output:

[mikkamp]

Update: A codecov report has been added in #1915
So far this only covers PHP, we also do unit testing in JS which we could include in the coverage report.