Improve code coverage reporting

[stevegrunwell]

Updated composer test-coverage to include a summary in the command-line test runner, as well as checks to ensure Xdebug is enabled in VVV.

Example output:

$ composer test-coverage
> bin/generate-test-coverage.sh

Enabling Xdebug in order to generate test coverage:
php7.0-fpm stop/waiting
php7.0-fpm start/running, process 728

Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml

Not running ajax tests. To execute these, use --group ajax.
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.
PHPUnit 5.7.21 by Sebastian Bergmann and contributors.

............................................                      44 / 44 (100%)

Time: 22.99 seconds, Memory: 32.00MB

OK (44 tests, 77 assertions)

Generating code coverage report in HTML format ... done


Code Coverage Report:
  2017-08-23 15:34:35

 Summary:
  Classes:        (0/0)
  Methods:        (0/0)
  Lines:   100.00% (103/103)

Additional code coverage details:
/srv/www/liquidweb-default/public_html/wp-content/plugins/sticky-tax/tests/coverage/