Add ui tests for --env option

Author: GuillaumeGomez

Diff for: tests/ui/extenv/extenv-env-overload.rs

@@ -0,0 +1,9 @@
+// run-pass
+// rustc-env:MY_VAR=tadam
+// compile-flags: --env MY_VAR=123abc -Zunstable-options
+
+// This test ensures that variables provided with `--env` take precedence over
+// variables from environment.
+fn main() {
+    assert_eq!(env!("MY_VAR"), "123abc");
+}

Diff for: tests/ui/extenv/extenv-env.rs

@@ -0,0 +1,5 @@
+// compile-flags: --env FOO=123abc -Zunstable-options
+// run-pass
+fn main() {
+    assert_eq!(env!("FOO"), "123abc");
+}

Diff for: tests/ui/extenv/extenv-not-env.rs

@@ -0,0 +1,7 @@
+// run-pass
+// rustc-env:MY_ENV=/
+// Ensures that variables not defined through `--env` are still available.
+
+fn main() {
+    assert!(!env!("MY_ENV").is_empty());
+}