Upgrade phpunit/php-code-coverage to version 9.0+

[thekid]

...achieving PHP 8 compatibility, see issue #9

[thekid]

Hrm, this breaks PHP 7.0, 7.1 and 7.2 builds: phpunit/php-code-coverage 9.2.0 requires php >=7.3, see https://travis-ci.org/github/xp-forge/coverage/jobs/732487095

[thekid]

Code Coverage BC breaks

Version 9 contains quite a bit of BC breaks, see https://github.com/sebastianbergmann/php-code-coverage/blob/master/ChangeLog.md#900---2020-08-07

The call to construct a CodeCoverage can be made compatible with versions 5, 6, 7 and 8 as well as version 9:

$filter= new Filter();

// Version 9, can be detected via class_exists(Selector::class)
$driver= (new Selector())->forLineCoverage($filter);

// Version 6, 7, 8: makes CodeCoverage use its private selectDriver() method
$driver= null;

$coverage= new CodeCoverage($driver, $filter);

Report method names have changed away from a get... style:

// Before
$report->getNumExecutedLines();
$report->getNumExecutableLines();
$report->getClasses();

// After
$report->numberOfExecutedLines();
$report->numberOfExecutableLines();
$report->classes();

For filters, the verbs have changed completely:

// Before
$filter->addDirectoryToWhitelist($folder);
$filter->hasWhitelist();

// After
$filter->includeDirectory($folder);
$filter->isEmpty();