minor #8925 Added a tip about tests and BCrypt (javiereguiluz)

This PR was squashed before being merged into the 3.3 branch (closes #8925).

Discussion
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Added a tip about tests and BCrypt

Related to symfony/demo#737.

Commits
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af8537a Added a tip about tests and BCrypt

Author: javiereguiluz

reference/configuration/security.rst

@@ -612,6 +612,12 @@ persisting the encoded password alone is enough.
     All the encoded passwords are ``60`` characters long, so make sure to
     allocate enough space for them to be persisted.
 
+.. tip::
+
+    A simple technique to make tests much faster when using BCrypt is to set
+    the cost to ``4``, which is the minimum value allowed, in the ``test``
+    environment configuration.
+
     .. _reference-security-firewall-context:
 
 Firewall Context