Make `compiletest` build `std` when it runs

[artemagvanian]

I am currently developing a lint that depends on all MIR from inside std to be accessible from the current compilation unit. My lint is packaged using dylint, which uses compiletest under the hood for testing. Hence, I am wondering if it is possible to tell compiletest to build std when it runs, making it do something similar to what cargo ... -Z build-std achieves.