Rustpkg needs to be testable

[graydon]

Sub-bug of #5677

Currently rustpkg is not testable. We need a test driver for it that sets up test packages and tries to perform various actions with them. This should include at minimum:

• a git-sourced package (filesystem-local repo is ok)
• a tarball / curl package (likewise)
• building, installing, testing, running a binary
• multiple workspaces and RUST_PATH
• single-crate library and binary packages
• multi-crate library and binary packages
• dependencies between multiple packages
• use of pkg.rs custom build logic
• building a C library

[catamorphism]

We agreed to have #[test]s that shell out with program_output to call rustpkg, for now. Eventually we'll be rewriting compiletest to use workcache and can repurpose that.

[catamorphism]

#5683 partly solves this, but eventually we still want to have a test runner that doesn't spawn separate processes. In addition, tests for building a C library, custom build logic, multi-crate packages, multiple workspaces, RUST_PATH, testing, running, and tarball/curl packages still need to be added.

[catamorphism]

I'm going to split this up into separate issues

[catamorphism]

C libraries: #6403
Custom build logic: #7239
Multi-crate packages: #7240
Multiple workspaces: #7241
RUST_PATH: #5682
rust test and rust run: #7242
tarball/curl packages: #7243

[catamorphism]

Closing in favor of individual issues