Test modification of a task by an on-add hook (test case for #3416) (#3443)

* Add test case to cover https://github.com/GothenburgBitFactory/taskwarrior/issues/3416

* Add (builtin) on-add-modify test hook and use it

* TDB2::add() move hook invocation before save (#3416)

test/test_hooks/on-add-modify

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Input:
+# - Line of JSON for proposed new task.
+read new_task
+
+if (echo $new_task | grep -qE '[tT]eh');
+then
+  new_task=$(echo $new_task | sed -r 's/([tT])eh/\1he/g')
+fi
+
+# Output:
+# - JSON, modified
+# - Optional feedback/error.
+echo $new_task
+echo 'FEEDBACK'
+
+# Status:
+# - 0:     JSON accepted, non-JSON is feedback.
+# - non-0: JSON ignored, non-JSON is error.
+exit 0