test/mpi/threads: insert MTest_thread_yield() in busy loops

This patch adds MTest_thread_yield() to tests that I missed in
pmodels#4421.

Note this does not affect the program behavior if OS-level threads (Pthreads)
are used because MTest_thread_yield() is mapped to no-op.

test/mpi/threads/pt2pt/greq_test.c

@@ -92,6 +92,7 @@ int main(int argc, char *argv[])
     flag = 0;
     while (!flag) {
         MPI_Test(&request, &flag, &status);
+        MTest_thread_yield();
     }
     MTest_Join_threads();
 
@@ -102,6 +103,7 @@ int main(int argc, char *argv[])
     outcount = 0;
     while (!outcount) {
         MPI_Testsome(1, &request, &outcount, indices, &status);
+        MTest_thread_yield();
     }
     MTest_Join_threads();
 
@@ -112,6 +114,7 @@ int main(int argc, char *argv[])
     flag = 0;
     while (!flag) {
         MPI_Testall(1, &request, &flag, &status);
+        MTest_thread_yield();
     }
     MTest_Join_threads();
 

test/mpi/threads/pt2pt/threaded_sr.c

@@ -118,7 +118,8 @@ int main(int argc, char *argv[])
     }
 
     /* Loop until the send flag is set */
-    while (sendok == -1);
+    while (sendok == -1)
+        MTest_thread_yield();
     if (!sendok) {
         errs++;
     }