SERVER-45048 added --separate-debug for unittests, and this can be seen in the unittest compilation logs, eg:

[2020/03/10 07:34:48.693] Generating debug info for build/cached/mongo/db/repl/db_repl_test into build/cached/mongo/db/repl/db_repl_test.debug
[2020/03/10 07:35:58.410] Stripping debug info from build/cached/mongo/db/repl/db_repl_test and adding .gnu.debuglink to build/cached/mongo/db/repl/db_repl_test.debug

However, "gather failed unittests" does not expect to have to collect separate foo.debug files, which means that the uploaded unittest files contain only the stripped binary, and not the (now separate) debug symbols. This isn't great for debugging, since the debug symbols are almost always needed for unittests.

Presumably the reason for adding --separate-debug, when switching to hygenic builds, was to avoid copying very large unittests binaries, and this concern is likely still valid. Therefore "gather failed unittests" should be updated to also grab the .debug files (which are likely still in the build tree, not the install tree).