[CI] Store functional tests duration with ms resolution in Junit

Summary:
This diffs make the time tracked as fractional seconds instead of
integral seconds, and add helpers to convert the result to the expected
resolution.

Test Plan:

./test/functional/test_runner.py -J junit.xml

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6983