Don't include __rust_drop_panic when testing libstd

Author: Amanieu

src/libstd/panicking.rs

@@ -58,6 +58,7 @@ extern "C" {
 /// This function is called by the panic runtime if FFI code catches a Rust
 /// panic but doesn't rethrow it. We don't support this case since it messes
 /// with our panic count.
+#[cfg(not(test))]
 #[rustc_std_internal_symbol]
 extern "C" fn __rust_drop_panic() -> ! {
     rtabort!("Rust panics must be rethrown");