Remove warning from spawn tests.

tests/spawnanytest.c

@@ -10,7 +10,7 @@
 int
 main(int argc, const char *argv[])
 {
-    char *program;
+    const char *program;
     int pid[10];
     int status;
 

tests/spawnmultipletest.c

@@ -11,7 +11,7 @@ int
 main(int argc, const char *argv[])
 {
     int origpid[10], pid[10];
-    char *program;
+    const char *program;
     int status;
 
     printf("Spawn parallel test wait any: ");

tests/spawnsingletest.c

@@ -11,7 +11,7 @@ int
 main(int argc, const char *argv[])
 {
     int origpid, pid;
-    char *program;
+    const char *program;
     int status;
 
     printf("Spawn parallel test wait any: ");