[DI] Improve integration test failue output (#5611)

If for some reason the Dynamic Instrumentation integration tests fail in
the `beforeEach` stage, there's a risk that some of the data structures
that the `afterEach` block depends on have not been populated.

To not spam the logs with these errors that are only present because of
previous errors, better use optional chaining when referencing these
data structures.

Author: watson

integration-tests/debugger/utils.js

@@ -109,8 +109,8 @@ function setup ({ env, testApp, testAppSource } = {}) {
   })
 
   afterEach(async function () {
-    t.proc.kill()
-    await t.agent.stop()
+    t.proc?.kill()
+    await t.agent?.stop()
   })
 
   return t