Add a test for stmt assign breaking in preview mode (#7516)

In preview mode, black will consistently break the right side first.
This doesn't work yet, but we'll need the test later.

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/assign_breaking.options.json

@@ -0,0 +1,8 @@
+[
+  {
+    "preview": "disabled"
+  },
+  {
+    "preview": "enabled"
+  }
+]

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/assign_breaking.py

@@ -0,0 +1,34 @@
+# Below is black stable style
+# In preview style, black always breaks the right side first
+
+if True:
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
+        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
+    ] = cccccccc.ccccccccccccc.cccccccc
+
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
+        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
+    ] = cccccccc.ccccccccccccc().cccccccc
+
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
+        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
+    ] = cccccccc.ccccccccccccc(d).cccccccc
+
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
+        cccccccc.ccccccccccccc(d).cccccccc + e
+    )
+
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
+        cccccccc.ccccccccccccc.cccccccc + e
+    )
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = (
+        cccccccc.ccccccccccccc.cccccccc
+        + eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
+    )
+
+    self._cache: dict[
+        DependencyCacheKey, list[list[DependencyPackage]]
+    ] = collections.defaultdict(list)
+    self._cached_dependencies_by_level: dict[
+        int, list[DependencyCacheKey]
+    ] = collections.defaultdict(list)