Fix Build: conditionally_existent_test in folly/test/Makefile.am

Summary:
[Folly] Fix Build: `conditionally_existent_test` in `folly/test/Makefile.am`.

HT: https://github.com/shindo.

Reviewed By: meyering

Differential Revision: D2910782

fb-gh-sync-id: 9593f026e7c58b8644abc5996da65005bcc2d095

folly/test/Makefile.am

@@ -159,7 +159,7 @@ string_test_SOURCES = StringTest.cpp
 string_test_LDADD = libfollytestmain.la
 TESTS += string_test
 
-conditionally_existent_test_SOURCES = ConditionallyExistent.cpp
+conditionally_existent_test_SOURCES = ConditionallyExistentTest.cpp
 conditionally_existent_test_LDADD = libfollytestmain.la
 TESTS += conditionally_existent_test