test: add "Debug" build to slow builds

Summary:
With a Debug build on CI, the feature_assumeutxo.py functional test (D17931) consistently times out when restarting the node with -reindex=1
Doubling the timeout factor fixes this.

Test Plan:

cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check-functional

Check the output of the cmake command for the message "Slow build detected, doubling timeout for check-functional...`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17945