Support multiple --target arguments

[sunshowers]

Rust 1.64 includes support for multiple --target or build.target arguments. Nextest should support them.

Some challenges:

1. Cargo's JSON messages don't contain the name of the target. However, it can be extracted from the relative path in detect_base_output_dir.
2. We need to extend the platform() predicate to handle specific targets. Maybe something like platform(target) to select all targets, and platform(target:<triple>) for a specific target.
3. Metadata serialization will need to account for these additional cases.

[sunshowers]

#538 has a WIP impl for this.