Add dependencies used by PackageTests to exe:cabal-tests

The runner allows the tests to use extra dependencies and the custom Prelude
from 'cabal-testsuite'.
However, if the tests use a dependency, say 'directory', and there are two
packages with the same unit id available in the store, the test fails since
it doesn't know which one to pick.
By including an extra dependency to directory, we force the test runner to
use a specific version directory, fixing the test failure.

cabal-testsuite/cabal-testsuite.cabal

@@ -101,6 +101,18 @@ executable cabal-tests
     , transformers
     -- dependencies specific to exe:cabal-tests
     , clock                 ^>= 0.7.2 || ^>=0.8
+    -- Extra dependencies used by PackageTests.
+    --
+    -- The runner allows the tests to use extra dependencies and the custom Prelude
+    -- from 'cabal-testsuite'.
+    -- However, if the tests use a dependency, say 'directory', and there are two
+    -- packages with the same unit id available in the store, the test fails since
+    -- it doesn't know which one to pick.
+    -- By including an extra dependency to directory, we force the test runner to
+    -- use a specific version directory, fixing the test failure.
+    --
+    -- See issue description and discussion: https://github.com/haskell/cabal/issues/8356
+    , directory
 
   build-tool-depends: cabal-testsuite:setup
   default-extensions: TypeOperators