Document run-tests workers flag (#103)

Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>

Author: Danack

Book/tests/running_the_test_suite.rst

@@ -90,7 +90,17 @@ You can specify multiple target folders or files to run by separating each path
 Executing tests in parallel
 ---------------------------
 
-You can't.
+Since PHP 7.4 you are able to run tests in parallel by passing a ``-j<workers>`` flag, where workers is the number of parallel processes you want. e.g. ``-j4`` would run 4 tests at once.
+
+Example for running tests through run-tests.php:
+```
+run-tests.php -j8
+```
+
+Example for running tests through make:
+```
+make test TESTS="-j8"
+```
 
 More options
 ------------