Add test oracles (#1800)

* Update 103-test-oracles.md

- Add definition for Test Oracle taken directly from ISTQB Glossary entry for subject.
- Include link to glossary entry under `Read` badge in free content section

* Update content/roadmaps/113-qa/content/100-qa-basics/103-test-oracles.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/113-qa/content/100-qa-basics/103-test-oracles.md

@@ -1 +1,6 @@
-# Test oracles
+# Test Oracles
+
+A test oracle is a mechanism; different from the program itself that can be used to check the correctness of the program's output for the test cases. Conceptually, we can consider testing a process in which the test cases are given to the test oracle and the program under testing.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://ecomputernotes.com/software-engineering/what-are-test-oracles'>What are Test Oracles</BadgeLink>