Copy properties back onto process.env rather than overwriting it

Copy the properties rather than directly overwriting `process.env` to
preserve behaviour like aliasing `process.env.PATH` with
`process.env.Path` on Windows.

Author: henrymercer

lib/testing-utils.js

@@ -83,7 +83,13 @@ export function setupTests(test: TestFn<any>) {
     sinon.restore();
 
     // Undo any modifications to the env
-    process.env = t.context.env;
+    // Copy the properties rather than directly overwriting `process.env` to
+    // preserve behaviour like aliasing `process.env.PATH` with
+    // `process.env.Path` on Windows.
+    for (const key of Object.keys(process.env)) {
+      delete process.env[key];
+    }
+    Object.assign(process.env, t.context.env);
   });
 }