Fix LPE test sensitivity to system fonts list

The Measure LPE tests only pass if the text labels have exactly the right size, which in particular only happens if the system decides to resolve "Sans" to "DejaVu Sans". This can fail if the system has other sans fonts installed such as Noto Sans. Therefore, we modify the files in the LPE test suite to explicitly request that the labels are set in DejaVu Sans. This makes the LPE test suite more robust against system differences, and resolves certain test failures that only happen outside of CI.

Related: #3554 (comment 1236682957)