It doesn’t work.
Time before ramdisk: 32.24 – 33.7 seconds
Time after ramdisk: 32.38 – 33.65 seconds
I am being unfair. The first run without the ramdisk was about 10 seconds slower, then the times were in the range above. With the ramdisk, it went straight into that range. Nevertheless, for the case in which I care most about test runtime, the change-stuff-and-run-again case, as long as the caches stay hot I’m not going to win anything by stuffing around with a ramdisk.