Clean up some <stdio.h> constants.

And actually test an assertion rather than just state it in a comment.

Bug: N/A
Test: ran tests
Change-Id: I07699483aca4aac4e089d8b99123cb5bde9b3c63
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index dac7056..1bb97a3 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1918,3 +1918,8 @@
   ASSERT_EQ(0, fclose(fp));
   AssertFileIs(tf.filename, "0123456789xxx");
 }
+
+TEST(STDIO_TEST, constants) {
+  ASSERT_LE(FILENAME_MAX, PATH_MAX);
+  ASSERT_EQ(L_tmpnam, PATH_MAX);
+}