Disable panic tests on Windows

Miri currently does not support `GetProcAddress`
and `GetModuleHandleW`, both of which end up getting invoked by the
libstd panic hook.

tests/run-pass/catch_panic.rs

@@ -1,3 +1,4 @@
+// ignore-windows: Unwind panicking does not currently work on Windows
 use std::panic::catch_unwind;
 use std::cell::Cell;
 

tests/run-pass/panic1.rs

@@ -1,3 +1,4 @@
+// ignore-windows: Unwind panicking does not currently work on Windows
 fn main() {
     panic!("Miri panic!");
 }

tests/run-pass/panic1.stderr

@@ -1 +1 @@
-thread 'main' panicked at 'Miri panic!', $DIR/panic1.rs:2:5
+thread 'main' panicked at 'Miri panic!', $DIR/panic1.rs:3:5

tests/run-pass/panic2.rs

@@ -1,3 +1,4 @@
+// ignore-windows: Unwind panicking does not currently work on Windows
 fn main() {
     let val = "Value".to_string();
     panic!("Miri panic with value: {}", val);

tests/run-pass/panic2.stderr

@@ -1 +1 @@
-thread 'main' panicked at 'Miri panic with value: Value', $DIR/panic2.rs:3:5
+thread 'main' panicked at 'Miri panic with value: Value', $DIR/panic2.rs:4:5