Add a unit test for already-fixed printf -0.0 formatting.

This was fixed by the upgrade to upstream head.

Bug: 5084292
Change-Id: Ia3bda1c0bbe38f428e22213b8bdbdf1a16caccf2

tests/stdio_test.cpp

@@ -400,6 +400,13 @@ TEST(stdio, snprintf_e) {
   EXPECT_STREQ("1.500000e+00", buf);
 }
 
+TEST(stdio, snprintf_negative_zero_5084292) {
+  char buf[BUFSIZ];
+
+  snprintf(buf, sizeof(buf), "%f", -0.0);
+  EXPECT_STREQ("-0.000000", buf);
+}
+
 TEST(stdio, popen) {
   FILE* fp = popen("cat /proc/version", "r");
   ASSERT_TRUE(fp != NULL);