Test that /dev/stdin, /dev/stdout, and /dev/stderr are all present.

Bug: http://b/31824379
Test: ran tests
Change-Id: I16034106eb61b9f32c9afef512c75f0b73c179b5
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index 844a9c0..d93c63f 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -2506,3 +2506,14 @@
 
   fclose(fp);
 }
+
+TEST(STDIO_TEST, dev_std_files) {
+  // POSIX only mentions /dev/stdout, but we should have all three (http://b/31824379).
+  char path[PATH_MAX];
+  ASSERT_GT(readlink("/dev/stdin", path, sizeof(path)), 0);
+  ASSERT_STREQ("/proc/self/fd/0", path);
+  ASSERT_GT(readlink("/dev/stdout", path, sizeof(path)), 0);
+  ASSERT_STREQ("/proc/self/fd/1", path);
+  ASSERT_GT(readlink("/dev/stderr", path, sizeof(path)), 0);
+  ASSERT_STREQ("/proc/self/fd/2", path);
+}