minor #3153 Update .gitattributes to remove tests from "dist" (barryvdh)

This PR was squashed before being merged into the 2.x branch (closes #3153).

Discussion
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Update .gitattributes to remove tests from "dist"

This was proposed many times before, but considering symfony/symfony#33579 is now merged, I propose this again. I refer to that issue as for the reasoning, as it applies here also.

I did not add the `docs` directory as there was much discussion about that before (discussion in  #942)

Related: #942 #1776 #1981 #2292 #2799 2905

Commits
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5cde1f50 Update .gitattributes to remove tests from \"dist\"

.gitattributes

@@ -0,0 +1,2 @@
+/tests export-ignore
+/phpunit.xml.dist export-ignore