Prebuild tests
Build unit tests in the build stage, not as part of the test being run.
This lets qemu-armhf trudge through the test stage of the build.
[1/4] Generating build_rstests with a custom command.
Finished dev [unoptimized + debuginfo] target(s) in 1.13s
[3/4] Running all tests.
1/12 rstest OK 78.16 s