enable validation for a bunch of more tests

Author: RalfJung

tests/compile-fail/panic.rs

@@ -1,5 +1,3 @@
-// FIXME: Probably failing due to https://github.com/solson/miri/issues/296
-// compile-flags: -Zmir-emit-validate=0
 //error-pattern: the evaluated program panicked
 
 fn main() {

tests/compile-fail/zst2.rs

@@ -1,5 +1,3 @@
-// FIXME: Probably failing due to https://github.com/solson/miri/issues/296
-// compile-flags: -Zmir-emit-validate=0
 // error-pattern: the evaluated program panicked
 
 #[derive(Debug)]

tests/compile-fail/zst3.rs

@@ -1,5 +1,3 @@
-// FIXME: Probably failing due to https://github.com/solson/miri/issues/296
-// compile-flags: -Zmir-emit-validate=0
 // error-pattern: the evaluated program panicked
 
 #[derive(Debug)]

tests/run-pass-fullmir/hashmap.rs

@@ -1,5 +1,3 @@
-// FIXME: disable validation until we figure out how to handle <https://github.com/solson/miri/issues/296>.
-// compile-flags: -Zmir-emit-validate=0
 use std::collections::{self, HashMap};
 use std::hash::BuildHasherDefault;
 

tests/run-pass/btreemap.rs

@@ -1,6 +1,3 @@
-// mir validation can't cope with `mem::uninitialized::<SomeEnum>()`
-// compile-flags: -Zmir-emit-validate=0
-
 #[derive(PartialEq, Eq, PartialOrd, Ord)]
 pub enum Foo {
     A(&'static str),