Honestly, it should be impossible; in the absence of some weird configuration, cargo test does strictly more work than cargo build. :P Can you reproduce it and file a bug?
i suspect most of that time is link time. Possibly the linker in use is not very parallel, and so linking one big executable with cargo build takes longer than many smaller test executable whose linking can actually be made parallel?
Both numbers are for debug builds. I don't know why `cargo test` is faster but I appreciate it.
Incremental release builds with `cargo build --release` are even slower taking ~35s on the 1240p.