Add content for Functional Testing (#1038)

Add a description and resources for Functional Testing

content/roadmaps/101-backend/content/112-testing/102-functional-testing.md

@@ -1 +1,8 @@
-# Functional testing
+# Functional Testing
+
+Functional testing is where software is tested to ensure functional requirements are met. Usually, it is a form of black box testing in which the tester has no understanding of the source code; testing is performed by providing input and comparing expected/actual output.
+It contrasts with non-functional testing, which includes performance, load, scalability, and penetration testing.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.guru99.com/functional-testing.html'>What is Functional Testing?</BadgeLink>
+<BadgeLink badgeText='Watch' href='https://youtu.be/j_79AXkG4PY'>Functional Testing vs Non-Functional Testing</BadgeLink>