Replace reftest with a problematic file name with a unit test.

Fix #7609, "error: unable to create file tests/ref/hello_a?foo#bar.html
(Invalid argument)" during git checkout on Windows.

Behavior change: passing an nonexistent file name on the command line
now shows a blank page (like network errors)
rather than exit with an error message.

tests/unit/util/lib.rs

@@ -14,3 +14,4 @@ extern crate euclid;
 #[cfg(test)] mod vec;
 #[cfg(test)] mod mem;
 #[cfg(test)] mod str;
+#[cfg(test)] mod opts;