Ruleset::setSniffProperty(): add test for edge case / unused sniff

I haven't been able to come up with a _real_ situation in which [this condition](https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/15db7232015d4fc1e023ef1eff0e65777a906f2c/src/Ruleset.php#L1476-L1479) could result in an early return - either via reading an XML ruleset or via inline `phpcs:set` annotation. Having said that, the method is `public` and is regularly used in test frameworks for external standards, so this code should remain in place.

This commit now safeguards the behaviour via a test.

tests/Core/Ruleset/SetSniffPropertyTest.php

@@ -202,6 +202,39 @@ public function testSetPropertyDoesNotThrowErrorOnInvalidPropertyWhenSetForCateg
     }//end testSetPropertyDoesNotThrowErrorOnInvalidPropertyWhenSetForCategory()
 
 
+    /**
+     * Test that attempting to set a property for a sniff which isn't registered will be ignored.
+     *
+     * @return void
+     */
+    public function testDirectCallIgnoredPropertyForUnusedSniff()
+    {
+        $sniffCode  = 'Generic.Formatting.SpaceAfterCast';
+        $sniffClass = 'PHP_CodeSniffer\\Standards\\Generic\\Sniffs\\Formatting\\SpaceAfterCastSniff';
+
+        // Set up the ruleset.
+        $config  = new ConfigDouble(['--standard=PSR1']);
+        $ruleset = new Ruleset($config);
+
+        $ruleset->setSniffProperty(
+            $sniffClass,
+            'ignoreNewlines',
+            [
+                'scope' => 'sniff',
+                'value' => true,
+            ]
+        );
+
+        // Verify that there are sniffs registered.
+        $this->assertGreaterThan(0, count($ruleset->sniffCodes), 'No sniff codes registered');
+
+        // Verify that our target sniff has NOT been registered after attempting to set the property.
+        $this->assertArrayNotHasKey($sniffCode, $ruleset->sniffCodes, 'Unused sniff was registered in sniffCodes, but shouldn\'t have been');
+        $this->assertArrayNotHasKey($sniffClass, $ruleset->sniffs, 'Unused sniff was registered in sniffs, but shouldn\'t have been');
+
+    }//end testDirectCallIgnoredPropertyForUnusedSniff()
+
+
     /**
      * Test that setting a property via a direct call to the Ruleset::setSniffProperty() method
      * sets the property correctly when using the new $settings array format.