test: Clean out the test suite a bit

This updates a number of ignore-test tests, and removes a few completely
outdated tests due to the feature being tested no longer being supported.

This brings a number of bench/shootout tests up to date so they're compiling
again. I make no claims to the performance of these benchmarks, it's just nice
to not have bitrotted code.

Closes #2604
Closes #9407
auto merge of #12457 : alexcrichton/rust/fix-some-tests, r=brson

This updates a number of ignore-test tests, and removes a few completely
outdated tests due to the feature being tested no longer being supported.

This brings a number of bench/shootout tests up to date so they're compiling
again. I make no claims to the performance of these benchmarks, it's just nice
to not have bitrotted code.

Closes #2604
Closes #9407