Auto merge of #14896 - g1smo:tidy-assignments, r=jdm

Tidy assignments

<!-- Please describe your changes on the following line: -->
I've added a new tidy rule (no = in the beginning of line) + tests for it. Also cleaned up a few rust source files to accord with the new rule.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14890 (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/layout/block.rs

@@ -1283,8 +1283,8 @@ impl BlockFlow {
                             content_block_size,
                             available_block_size));
 
-                    candidate_block_size_iterator.candidate_value
-                        = solution.unwrap().block_size;
+                    candidate_block_size_iterator.candidate_value =
+                        solution.unwrap().block_size;
                 }
             }
         }

components/layout/construct.rs

@@ -1290,8 +1290,8 @@ impl<'a, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode>
         for kid in node.children() {
             // CSS 2.1 § 17.2.1. Treat all non-column child fragments of `table-column-group`
             // as `display: none`.
-            if let ConstructionResult::ConstructionItem(ConstructionItem::TableColumnFragment(fragment))
-                   = kid.swap_out_construction_result() {
+            if let ConstructionResult::ConstructionItem(ConstructionItem::TableColumnFragment(fragment)) =
+                kid.swap_out_construction_result() {
                 col_fragments.push(fragment)
             }
         }