Add support for retrying failed tests to nextest

[sunshowers]

This needs to be done with a little care:

• The initial, simpler use requires keeping track of which Cargo arguments the build was run with -- this is relatively simple.
• An extended use case to solve is "what if the user wants to grow the set of tests or binaries that are run?" This is going to require us keeping track of the exact set of binaries run and Cargo arguments passed. Probably worth discussing with some folks before doing so.

Overall this is a stateful operation, kind of like a source control bisect.