Investigate splitting 32bit windows tests

[aidanhs]

https://mozilla.logbot.info/rust-infra/20180130#c14213927-c14214012

It'd be good to get some notes on how this might be done in rustbuild.

A working implementation would be fine too :)

[aidanhs]

cc #46903

[Mark-Simulacrum]

I'll do my best to come up with some instructions this weekend. If someone is super interested and I haven't yet, ping me, because it'll help expedite matters.

[aidanhs]

Test splitting also useful for #48192 which is causing timeouts, though the root cause is different.