Add link to should_panic section in contract testing docs (#847)

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Closes #778

## Introduced changes

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- Added link to should_panic section in contract testing docs

## Breaking changes

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## Checklist

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- [x] Linked relevant issue
- [x] Updated relevant documentation
- [x] Added relevant tests
- [x] Performed self-review of the code
- [x] Added changes to `CHANGELOG.md`

Author: cptartur

docs/src/testing/contracts.md

@@ -183,3 +183,9 @@ Running 1 test(s) from src/
 [PASS] package_name::handling_errors
 Tests: 1 passed, 0 failed, 0 skipped
 ```
+
+### Expecting Test Failure
+
+Sometimes the test code failing can be a desired behavior.
+Instead of manually handling it, you can simply mark your test as `#[should_panic(...)]`.
+[See here](./testing.md#expected-failures) for more details.