Update coding style with test method annotation (facebookincubator#2194)

Summary:
Update coding style with test method annotation, e.g. if we want to add a test method say 'foo' to an object, we will name it with prefix TEST, TEST_foo.

Pull Request resolved: facebookincubator#2194

Reviewed By: pedroerp, mbasmanova

Differential Revision: D38372050

Pulled By: xiaoxmeng

fbshipit-source-id: df05b6c386c9c2217cb565080f93386a5a755f80

CODING_STYLE.md

@@ -57,7 +57,11 @@ line width, indentation and ordering (for includes, using directives and etc). 
 * Use **snake_case** for namespace names and build targets
 * Use **UPPER_SNAKE_CASE** for macros.
 * Use **kPascalCase** for static constants and enumerators.
-
+* Use **testing** prefix for test only class methods, e.g. obj.testingFoo().
+  As much as possible, refrain from adding test methods, and test objects using
+  their public APIs. Only use test methods in rare edge cases where this is not
+  possible. For example, MemoryAllocator::testingSetFailureInjection() is used
+  to to inject various memory allocation failures to test error handling paths.
 
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