Cleanup ftruncate negative test.

Addresses post-submit comments from
I54692ab8105dd09db6af7a2c0894a17bdd118aa0.

Bug: http://b/21309901
Change-Id: Ie19ee8cdcc4055a65fe7dfc103156e54eafe8977
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index a21578a..79c16d7 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -179,10 +179,8 @@
 TEST(unistd, ftruncate_negative) {
   TemporaryFile tf;
   errno = 0;
-  int rc = ftruncate(tf.fd, -123);
-  int err = errno;
-  ASSERT_EQ(-1, rc);
-  ASSERT_EQ(EINVAL, err);
+  ASSERT_EQ(-1, ftruncate(tf.fd, -123));
+  ASSERT_EQ(EINVAL, errno);
 }
 
 static bool g_pause_test_flag = false;