posix: fix stropts_h.c faulty imports and function usage

tests/posix/headers/src/stropts_h.c

@@ -1,6 +1,5 @@
-#include <ztest.h>
 #include <zephyr/posix/stropts.h>
-#include <stropts.h>
+#include "_common.h"
 /**
  * @brief Test existence and basic functionality of stropts.h
  *
@@ -14,12 +13,3 @@ ZTEST(posix_headers, test_stropts_h)
 	zassert_true(sizeof(((struct strbuf *)0)->len) > 0, "len size is 0");
 	zassert_true(sizeof(((struct strbuf *)0)->buf) > 0, "buf size is 0");
 }
-
-void test_main(void)
-{
-	ztest_test_suite(posix_stropts_test,
-		ztest_unit_test(test_stropts_h)
-	);
-
-	ztest_run_test_suite(posix_stropts_test);
-}