Run extended tests when there are changes to datafusion-testing pin (#18310)

## Which issue does this PR close?



## Rationale for this change

The `extended` tests rely on the checkout of datafusion-testing (that
has the expected results for the sqlite sqllogictest suite)

However, we don't currently run the extended tests when that pin is
changed so we could potentially break CI on main if we don't catch
changes in code review (this just happened to me in
#17866 (review))

## What changes are included in this PR?

1. Run extended CI tests on changes to datafusion-testing 



## Are these changes tested?

I tested this in PR
- #18311
- #18312

## Are there any user-facing changes?
No

Author: alamb

.github/workflows/extended.yml

@@ -44,6 +44,7 @@ on:
       - 'datafusion/physical*/**/*.rs'
       - 'datafusion/expr*/**/*.rs'
       - 'datafusion/optimizer/**/*.rs'
+      - 'datafusion-testing'
   workflow_dispatch:
     inputs:
       pr_number: