Test to compare generated code against expected snapshots, and easy way to update snapshots

[dtolnay]

#231 implemented a couple basic code generator tests just based on str::contains like this:

cxx/gen/src/mod.rs

Lines 89 to 91 in 447c808

	// To avoid continual breakage we won't test every byte.
	// Let's look for the major features.
	assert!(output.contains("void cxxbridge03$do_cpp_thing(::rust::Str::Repr foo)"));

This is fine on a small scale but as we flesh out more of this kind of test, it's going to be better to have tests that can be programmatically kept up to date. Like a directory of input source files and the complete output of the code generator against those files, with a single script to update them all when iterating on the code generator (or instead of a script, maybe we run the test with a specific env var to overwrite the snapshots, as https://github.com/dtolnay/trybuild does).

This is how the Rust compiler does its MIR tests (https://github.com/rust-lang/rust/blob/6b269e44322cfca727fd0e793d3a60bd371cbcae/src/test/mir-opt/README.md). x.py test --bless updates them all, and we can inspect the generated diff to tell whether the changes are as intended.