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\('

auth/oauth2/tests/auth_test.php

@@ -46,8 +46,8 @@ class auth_oauth2_auth_testcase extends advanced_testcase {
                 ['subject', 'message'],
                 array_keys($info),
                 '', 0.0, 10, true);
-        $this->assertContains(
+        $this->assertStringContainsString(
                 'your password cannot be reset because you are using your account on another site to log in',
                 $info['message']);
     }
-}
+}