[libc++] Use add_lit_testsuite to register the libc++ test suite

The Runtimes build uses variables set by add_lit_testsuite to discover
tests suites to run.

Differential Revision: https://reviews.llvm.org/D97913

libcxx/test/CMakeLists.txt

@@ -103,7 +103,7 @@ if (LIBCXX_INCLUDE_TESTS)
     DEPENDS cxx ${LIBCXX_TEST_DEPS}
     COMMENT "Builds dependencies required to run the test suite.")
 
-  add_lit_target(check-cxx
+  add_lit_testsuite(check-cxx
     "Running libcxx tests"
     ${CMAKE_CURRENT_BINARY_DIR}
     DEPENDS cxx-test-depends