Add env var test variable in compiletest

tests/compiletest.rs

@@ -37,6 +37,9 @@ fn run_tests(mode: &str, path: &str, target: &str, mut flags: Vec<String>) {
         flags.push(format!("--sysroot {}", sysroot));
     }
 
+    // Add a test env var to do evironment communication tests
+    std::env::set_var("MIRI_ENV_VAR_TEST", "0");
+
     // The rest of the configuration.
     let mut config = compiletest::Config::default().tempdir();
     config.mode = mode.parse().expect("Invalid mode");

tests/run-pass/communication.rs

@@ -2,5 +2,5 @@
 // compile-flags: -Zmiri-enable-communication
 
 fn main() {
-    assert!(std::env::var("PWD").is_ok());
+    assert_eq!(std::env::var("MIRI_ENV_VAR_TEST"), Ok("0".to_owned()));
 }