internal: add simple smoke test for project model

Our project model code is rather complicated -- the logic for lowering
from `cargo metadata` to `CrateGraph` is fiddly and special-case. So
far, we survived without testing this at all, but this increasingly
seems like a poor option.

So this PR introduces a simple tests just to detect the most obvious
failures. The idea here is that, although we rely on external processes
(cargo & rustc), we are actually using their stable interfaces, so we
might just mock out the outputs.

Long term, I would like to try to virtualize IO here, so as to do such
mocking in a more principled way, but lets start simple.

Should we forgo the mocking and just call `cargo metadata` directly
perhaps? Touch question -- I personally feel that fast, in-process tests
are more important in this case than any extra assurance we get from
running the real thing.

Super-long term, we would probably want to extend our heavy tests to
cover more use-cases, but we should figure a way to do that without
slowing the tests down for everyone.

Perhaps we need two-tiered bors system, where we pull from `master` into
`release` branch only when an additional set of tests passes?

crates/project_model/src/lib.rs

@@ -24,6 +24,9 @@ mod workspace;
 mod rustc_cfg;
 mod build_scripts;
 
+#[cfg(test)]
+mod tests;
+
 use std::{
     convert::{TryFrom, TryInto},
     fs::{self, read_dir, ReadDir},