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/Encoder/Argon2iPasswordEncoderTest.php

@@ -21,7 +21,7 @@
  */
 class Argon2iPasswordEncoderTest extends TestCase
 {
-    const PASSWORD = 'password';
+    private const PASSWORD = 'password';
 
     protected function setUp(): void
     {

Tests/Encoder/BCryptPasswordEncoderTest.php

@@ -21,8 +21,8 @@
  */
 class BCryptPasswordEncoderTest extends TestCase
 {
-    const PASSWORD = 'password';
-    const VALID_COST = '04';
+    private const PASSWORD = 'password';
+    private const VALID_COST = '04';
 
     public function testCostBelowRange()
     {

Tests/Validator/Constraints/UserPasswordValidatorTest.php

@@ -23,8 +23,8 @@
  */
 abstract class UserPasswordValidatorTest extends ConstraintValidatorTestCase
 {
-    const PASSWORD = 's3Cr3t';
-    const SALT = '^S4lt$';
+    private const PASSWORD = 's3Cr3t';
+    private const SALT = '^S4lt$';
 
     /**
      * @var TokenStorageInterface