Auto merge of #12243 - TravisDean:else-brace-check, r=ConnorGBrewster

Add style check and test for else braces

<!-- Please describe your changes on the following line: -->
Added a regex check to enforce project style guidelines. Added unit check. Catches:
```
}
else {
```
when it should be
`} else {`

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors (no core files changed)
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12234 (github issue number if applicable).

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

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12243)
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components/script/dom/htmlmediaelement.rs

@@ -133,9 +133,8 @@ impl AsyncResponseListener for HTMLMediaElementContext {
 
             // Step 5
             elem.fire_simple_event("error");
-        }
-        // => "If the media data cannot be fetched at all..."
-        else {
+        } else {
+            // => "If the media data cannot be fetched at all..."
             elem.queue_dedicated_media_source_failure_steps();
         }
 

components/script/dom/xmlhttprequest.rs

@@ -1128,9 +1128,8 @@ impl XMLHttpRequest {
                 // Step 5
                 if self.response_type.get() == XMLHttpRequestResponseType::_empty {
                     return None;
-                }
-                // Step 6
-                else {
+                } else {
+                    // Step 6
                     temp_doc = self.document_text_html();
                 }
             },
@@ -1143,8 +1142,7 @@ impl XMLHttpRequest {
             Some(Mime(_, mime::SubLevel::Ext(sub), _)) => {
                 if sub.ends_with("+xml") {
                     temp_doc = self.handle_xml();
-                }
-                else {
+                } else {
                     return None;
                 }
             },