minor #38931 Set constant visibility in tests to private where possible (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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Set constant visibility in tests to private where possible

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

As discussed on Slack, this PR prepares the introduction of a CS fixer rule to enforce explicit visibility declarations for constants. I've checked class constants in test and fixture classes and if I only found usages within the same class, I've declared them `private`. This way, I also found a few constants that weren't used at all anymore, so I've removed those.

Commits
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43ce6dfe44 Set constant visibility in tests to private where possible.

Author: fabpot

Tests/Extension/Core/DataTransformer/BooleanToStringTransformerTest.php

@@ -16,7 +16,7 @@
 
 class BooleanToStringTransformerTest extends TestCase
 {
-    const TRUE_VALUE = '1';
+    private const TRUE_VALUE = '1';
 
     /**
      * @var BooleanToStringTransformer

Tests/Extension/Validator/ViolationMapper/ViolationMapperTest.php

@@ -32,10 +32,10 @@
  */
 class ViolationMapperTest extends TestCase
 {
-    const LEVEL_0 = 0;
-    const LEVEL_1 = 1;
-    const LEVEL_1B = 2;
-    const LEVEL_2 = 3;
+    private const LEVEL_0 = 0;
+    private const LEVEL_1 = 1;
+    private const LEVEL_1B = 2;
+    private const LEVEL_2 = 3;
 
     /**
      * @var EventDispatcherInterface