Cargo's test suite no longer works correctly

[brson]

Dynamic linking issues around PATH of course: rust-lang/cargo#3394

[retep998]

Rustup no longer adds the toolchain's bin folder to the path when running the toolchain binaries. Plugins dynamically link the standard library and as a result must be able to find the dlls from the toolchain's bin folder. When running tests on a plugin, the plugin test binary still depends on those dlls, but is running standalone, and because the dlls are no longer in PATH, the test binary fails to find the dlls and crashes.

This is a pretty major blocker for anyone trying to test plugins. Rocket is one such project affected by this. This also prevents anyone from using -Cprefer-dynamic.