Add content for Unit Testing (#1037)

Add a description and resources for Unit Testing

content/roadmaps/101-backend/content/112-testing/101-unit-testing.md

@@ -1 +1,7 @@
-# Unit testing
+# Unit Testing
+
+Unit testing is where individual __units__ (modules, functions/methods, routines, etc.) of software are tested to ensure their correctness. This low-level testing ensures smaller components are functionally sound while taking the burden off of higher-level tests. Generally, a developer writes these tests during the development process and they are run as automated tests.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.guru99.com/unit-testing-guide.html'>Unit Testing Tutorial</BadgeLink>
+<BadgeLink badgeText='Watch' href='https://youtu.be/3kzHmaeozDI'>What is Unit Testing?</BadgeLink>