Update testing.rst

Fix dead link, @see:
https://github.com/silexphp/Silex/issues/1020
minor #1021 Update testing.rst (1emming)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1021).

Discussion
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Update testing.rst

Fix dead link, @see:
https://github.com/silexphp/Silex/issues/1020

Commits
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8c8b6a6 Update testing.rst