minor symfony#29174 Add required key attribute (greg0ire)

This PR was merged into the 2.8 branch.

Discussion
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Add required key attribute

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I am getting warnings when running tests with recent phpunit versions (since 7.2.0) :

> `- Element 'element': The attribute 'key' is required but missing.`

This requirement is far from being new, what is recent is phpunit
validating its configuration file against the XSD schema.

See sebastianbergmann/phpunit@d4484be

This is pedantic (not a bugfix), and might be a bit painful to merge upstream, so if you feel like I should target a more recent instead and let the old branches go on with their lives, please tell me.

Commits
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c0733c2 Add required key attribute

Author: nicolas-grekas

phpunit.xml.dist

@@ -56,8 +56,8 @@
                 <array>
                     <element key="time-sensitive">
                         <array>
-                            <element><string>Symfony\Component\Console</string></element>
-                            <element><string>Symfony\Component\HttpFoundation</string></element>
+                            <element key="0"><string>Symfony\Component\Console</string></element>
+                            <element key="1"><string>Symfony\Component\HttpFoundation</string></element>
                         </array>
                     </element>
                 </array>