JSTests had be generating coverage, though unittests were missing for awhile, and https://jira.mongodb.org/browse/SERVER-102984 recently repaired that.

While debugging/focusing/fixing those, it was observed that JSTests stopped collecting coverage, sometime between March 17 and March 18. Given the gcov segfaults, the timing of some v4-v5 toolchain changes that went in in that window appear to be suspicious.