Add env exclusion test

Author: pvdrz

tests/run-pass/env-exclude.rs

@@ -0,0 +1,6 @@
+// ignore-windows: TODO env var emulation stubbed out on Windows
+// compile-flags: -Zmiri-disable-isolation -Zmiri-env-exclude=MIRI_ENV_VAR_TEST
+
+fn main() {
+    assert!(std::env::var("MIRI_ENV_VAR_TEST").is_err());
+}