Auto merge of #3309 - ChrisDenton:windows-rand, r=RalfJung

Test windows random shims

This adds tests for `BCryptGenRandom` and `RtlGenRandom` (aka `SystemFunction036`). Note that `BCryptGenRandom` comes in two flavours:

```rust
BCryptGenRandom(null_mut(), key.as_mut_ptr(), len, BCRYPT_USE_SYSTEM_PREFERRED_RNG)
BCryptGenRandom(BCRYPT_RNG_ALG_HANDLE, key.as_mut_ptr(), len, 0)
```

Fixes #3308

Author: bors

tests/pass/shims/windows-rand.rs

@@ -0,0 +1,41 @@
+//@only-target-windows: this directly tests windows only random functions
+use core::ffi::c_void;
+use core::mem::size_of_val;
+use core::ptr::null_mut;
+
+// Windows API definitions.
+type NTSTATUS = i32;
+type BOOLEAN = u8;
+const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
+const BCRYPT_RNG_ALG_HANDLE: *mut c_void = 0x81 as *mut c_void;
+#[link(name = "bcrypt")]
+extern "system" {
+    fn BCryptGenRandom(
+        halgorithm: *mut c_void,
+        pbbuffer: *mut u8,
+        cbbuffer: u32,
+        dwflags: u32,
+    ) -> NTSTATUS;
+}
+#[link(name = "advapi32")]
+extern "system" {
+    #[link_name = "SystemFunction036"]
+    fn RtlGenRandom(RandomBuffer: *mut u8, RandomBufferLength: u32) -> BOOLEAN;
+}
+
+fn main() {
+    let mut key = [0u8; 24];
+    let len: u32 = size_of_val(&key).try_into().unwrap();
+    let ret = unsafe {
+        BCryptGenRandom(null_mut(), key.as_mut_ptr(), len, BCRYPT_USE_SYSTEM_PREFERRED_RNG)
+    };
+    // NTSTATUS codes use the high bit to indicate an error
+    assert!(ret >= 0);
+
+    let ret = unsafe { BCryptGenRandom(BCRYPT_RNG_ALG_HANDLE, key.as_mut_ptr(), len, 0) };
+    assert!(ret >= 0);
+
+    let ret = unsafe { RtlGenRandom(key.as_mut_ptr(), len) };
+    // RtlGenRandom returns a BOOLEAN where 0 indicates an error
+    assert_ne!(ret, 0);
+}