[CI] Don't run the electrum tests in parallel of the functional tests

Summary: This is a best effort to avoid port collision. Also the functional tests now use all the cores so better avoid adding too much load.

Test Plan:

./contrib/teamcity/build-configurations.py build-master

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17341