Fix elf/tst-dlmopen-twice not to exhaust static TLS

By default glibc only allocates enough static TLS for 4 link namespaces
including the initial one. So only use 3 dlmopens in the test.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

elf/tst-dlmopen-twice.c

@@ -46,8 +46,8 @@ do_test (void)
   recurse (1);
 
   /* Then with nesting.  The constant needs to be less than the
-     internal DL_NNS namespace constant.  */
-  recurse (10);
+     glibc.rtld.nns tunable (which is 4 by default).  */
+  recurse (3);
   return 0;
 }