Generic/NoSilencedErrors: improve test coverage

This commit adds two new tests to cover lines of the
Generic.PHP.NoSiliencedErrors that were not covered previously. One has
to do with a sniff property that allows users to set this sniff as an
error instead of a warning (default value) and the other with
calculating $contextLength when its value should be smaller than the
default value of 4.

Author: rodrigoprimo

src/Standards/Generic/Tests/PHP/NoSilencedErrorsUnitTest.inc

@@ -8,3 +8,9 @@ if (@in_array($array, $needle))
 }
 
 $hasValue = @in_array(haystack: $array, needle: $needle);
+
+// phpcs:set Generic.PHP.NoSilencedErrors error true
+$fileContent = @file_get_contents('non_existent_file.txt');
+// phpcs:set Generic.PHP.NoSilencedErrors error false
+
+echo @$undefinedVariable;

src/Standards/Generic/Tests/PHP/NoSilencedErrorsUnitTest.php

@@ -30,7 +30,7 @@ class NoSilencedErrorsUnitTest extends AbstractSniffUnitTest
      */
     public function getErrorList()
     {
-        return [];
+        return [13 => 1];
 
     }//end getErrorList()
 
@@ -48,6 +48,7 @@ public function getWarningList()
         return [
             5  => 1,
             10 => 1,
+            16 => 1,
         ];
 
     }//end getWarningList()