MDL-67673 phpunit: Remove deprecated assertContains() uses on strings

Both assertContains() and assertNotContains() are deprecated in PHPUnit 8
for operations on strings. Also the optional case parameter is. All uses
must be changed to one of:

- assertStringContainsString()
- assertStringContainsStringIgnoringCase()
- assertStringNotContainsString()
- assertStringNotContainsStringIgnoringCase()

More info: https://github.com/sebastianbergmann/phpunit/issues/3422

Regexp to find all uses:

ag 'assert(Not)?Contains\('

cache/tests/cache_test.php

@@ -1729,7 +1729,7 @@ class core_cache_testcase extends advanced_testcase {
             cache_helper::purge_by_definition('phpunit', 'purge2');
             $this->fail('Should not be able to purge a definition required identifiers without providing them.');
         } catch (coding_exception $ex) {
-            $this->assertContains('Identifier required for cache has not been provided', $ex->getMessage());
+            $this->assertStringContainsString('Identifier required for cache has not been provided', $ex->getMessage());
         }
     }