[TASK] Make the testcases final (#435)

This helps communicate that the testcases are not intended to
be subclassed.

Fixes #431

Author: oliverklee

tests/CSSList/AtRuleBlockListTest.php

@@ -11,7 +11,7 @@
 /**
  * @covers \Sabberworm\CSS\CSSList\AtRuleBlockList
  */
-class AtRuleBlockListTest extends TestCase
+final class AtRuleBlockListTest extends TestCase
 {
     /**
      * @test

tests/CSSList/DocumentTest.php

@@ -11,7 +11,7 @@
 /**
  * @covers \Sabberworm\CSS\CSSList\Document
  */
-class DocumentTest extends TestCase
+final class DocumentTest extends TestCase
 {
     /**
      * @var Document

tests/CSSList/KeyFrameTest.php

@@ -11,7 +11,7 @@
 /**
  * @covers \Sabberworm\CSS\CSSList\KeyFrame
  */
-class KeyFrameTest extends TestCase
+final class KeyFrameTest extends TestCase
 {
     /**
      * @var KeyFrame

tests/Comment/CommentTest.php

@@ -14,7 +14,7 @@
  * @covers \Sabberworm\CSS\OutputFormat
  * @covers \Sabberworm\CSS\OutputFormatter
  */
-class CommentTest extends TestCase
+final class CommentTest extends TestCase
 {
     /**
      * @test

tests/OutputFormatTest.php

@@ -11,7 +11,7 @@
 /**
  * @covers \Sabberworm\CSS\OutputFormat
  */
-class OutputFormatTest extends TestCase
+final class OutputFormatTest extends TestCase
 {
     /**
      * @var string

tests/ParserTest.php

@@ -35,7 +35,7 @@
  * @covers \Sabberworm\CSS\Value\Size::parse
  * @covers \Sabberworm\CSS\Value\URL::parse
  */
-class ParserTest extends TestCase
+final class ParserTest extends TestCase
 {
     /**
      * @test

tests/RuleSet/DeclarationBlockTest.php

@@ -10,7 +10,7 @@
 /**
  * @covers \Sabberworm\CSS\RuleSet\DeclarationBlock
  */
-class DeclarationBlockTest extends TestCase
+final class DeclarationBlockTest extends TestCase
 {
     /**
      * @param string $sCss

tests/RuleSet/LenientParsingTest.php

@@ -19,7 +19,7 @@
  * @covers \Sabberworm\CSS\Value\Size::parse
  * @covers \Sabberworm\CSS\Value\URL::parse
  */
-class LenientParsingTest extends TestCase
+final class LenientParsingTest extends TestCase
 {
     /**
      * @test

tests/Value/CalcRuleValueListTest.php

@@ -9,7 +9,7 @@
 /**
  * @covers \Sabberworm\CSS\Value\CalcRuleValueList
  */
-class CalcRuleValueListTest extends TestCase
+final class CalcRuleValueListTest extends TestCase
 {
     /**
      * @test