Issue #3504204: Set the HTTP_USER_AGENT environment variable by the Simpletest user agent

Closes #3504204

core/tests/Drupal/Nightwatch/Commands/drupalInstall.js

@@ -50,6 +50,9 @@ exports.command = function drupalInstall(
       value: encodeURIComponent(installData.user_agent),
       path: url.pathname,
     });
+    // Set the HTTP_USER_AGENT environment variable to detect the test
+    // environment in the command line.
+    process.env.HTTP_USER_AGENT = installData.user_agent;
   } catch (error) {
     this.assert.fail(error);
   }