Auto merge of #39851 - alexcrichton:verify-unstable, r=brson

test: Verify all sysroot crates are unstable

As we continue to add more crates to the compiler and use them to implement
various features we want to be sure we're not accidentally expanding the API
surface area of the compiler! To that end this commit adds a new `run-make` test
which will attempt to `extern crate foo` all crates in the sysroot, verifying
that they're all unstable.

This commit discovered that the `std_shim` and `test_shim` crates were
accidentally stable and fixes the situation by deleting those shims. The shims
are no longer necessary due to changes in Cargo that have happened since they
were originally incepted.

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