Fix asan.test_main_thread_em_asm_pthread (#21339)

Without this change this tests fails to compile with `em++: error:
-pthread + ALLOW_MEMORY_GROWTH may run non-wasm code slowly`.

This test was recently added in #21292 and this wasn't caught
immediately because we don't run the `asan` suite during CI.

test/test_core.py

@@ -1897,6 +1897,8 @@ def test_em_asm_2(self):
     'pthread': (['-pthread', '-sPROXY_TO_PTHREAD', '-sEXIT_RUNTIME'],),
   })
   def test_main_thread_em_asm(self, args):
+    if args:
+      self.setup_node_pthreads()
     src = read_file(test_file('core/test_em_asm_2.cpp'))
     create_file('test.cpp', src.replace('EM_ASM', 'MAIN_THREAD_EM_ASM'))