MDL-73278 phpunit: Fix namespaces for the renamed classes

Within the renamed classes in the previous commit, there were a
few incorrect namespace, this commit fixes all them to be
correct component[\api] ones.

Also fix a few incorrect leading backslashes in namespace/use
statements. And tests extending renamed tests.

course/tests/course_image_cache_test.php

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 
-namespace tests\core_course;
+namespace core_course;
 
 use context_user;
 use context_course;