doc: Remove outdated reference to "core" and front-end downloads

gcc/ChangeLog:

	* doc/install/testing.rst: Remove anachronism about separate
	source tarballs.

gcc/doc/install/testing.rst

@@ -22,9 +22,7 @@ but it can give you confidence in your new GCC installation or point out
 problems before you install and start using your new GCC.
 
 First, you must have :ref:`downloaded the testsuites <downloading-the-source>`.
-These are part of the full distribution, but if you downloaded the
-'core' compiler plus any front ends, you must download the testsuites
-separately.
+These are included in the source tarball.
 
 Second, you must have the testing tools installed.  This includes
 `DejaGnu <https://www.gnu.org/software/dejagnu/>`_, Tcl, and Expect;