This configures all `qa` projects to use the distribution contained in
the `tests.distribution` system property if it is set. The goal is to
create a simple way to run tests against the default distribution which
has x-pack basic features enabled while not forcing these tests on all
contributors. You run these tests by doing something like:
```
./gradlew -p qa -Dtests.distribution=zip check
```
or
```
./gradlew -p qa -Dtests.distribution=zip bwcTest
```
x-pack basic *shouldn't* get in the way of any of these tests but
nothing is ever perfect so this we have to disable a few when running
with the zip distribution.
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