[libc] Re-enable thrd_test.

Other threads related tests have been using the API that is tested here
without any problems. The main reason for disabling the test has also been
fixed many months ago.

Author: Siva Chandra Reddy

libc/test/src/threads/CMakeLists.txt

@@ -17,19 +17,18 @@ add_libc_unittest(
     libc.src.threads.thrd_join
 )
 
-# Disable thrd_test for now as it is flaky on the bots.
-# add_libc_unittest(
-#   thrd_test
-#   SUITE
-#     libc_threads_unittests
-#   SRCS
-#     thrd_test.cpp
-#   DEPENDS
-#     libc.include.threads
-#     libc.src.errno.__errno_location
-#     libc.src.threads.thrd_create
-#     libc.src.threads.thrd_join
-# )
+add_libc_unittest(
+  thrd_test
+  SUITE
+    libc_threads_unittests
+  SRCS
+    thrd_test.cpp
+  DEPENDS
+    libc.include.threads
+    libc.src.errno.__errno_location
+    libc.src.threads.thrd_create
+    libc.src.threads.thrd_join
+)
 
 add_libc_unittest(
   mtx_test