mk: Fix doc tests for multiple targets

It only really makes sense to run tests for the build target anyway because it's
not guaranteed that you can execute other targets.

This is blocking the next snapshot
Remove out of date test

The associated issue has been closed for 2 years!
auto merge of #11139 : alexcrichton/rust/fix-snapshots, r=luqmana

It only really makes sense to run tests for the build target anyway because it's
not guaranteed that you can execute other targets.

This is blocking the next snapshot
Method-ify CStore
auto merge of #11146 : sfackler/rust/cleanup, r=alexcrichton

I also deleted a test which has apparently not been needed for a very long time.