avoid double bundle registration

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | i hope so
| Fixed tickets | https://github.com/symfony-cmf/Testing/issues/51
| License       | MIT
| Doc PR        | -

As described in the issue, i got an exception when trying to use both doctrine bundles in one test. Cause the bundle sets do have common bundles to register. So we need to make the list of required bundles unique. But as there is no real point to have a simple array with some bundle names (bundles are instantiated directly after setting them and stored in the '$requiredBundles'  property of the kernel), i needed to take an other way: i filter the list of required bundles and skip an existing one.
I do not really know if the test pass, got some strange output locally.