Support running tests using RR

[firelizzard18]

It would be amazing if gotestsum supported running tests using rr. I've hacked something together to use with --raw-command but it's ugly. I am imagining something like gotestsum --engine rr (with the default being --engine go).

1. Update gotestsum to process packages separately
   • Detect if the caller passed a specific package, or something ending in /...
   • In the latter case, turn that into a list of packages
   • I used go list -f '{{if .TestGoFiles}}{{.ImportPath}}{{end}}' $1 to list only the packages containing test files
2. Run the test with rr
   • Add -exec 'rr record -o $TRACE' to the go test invocation
   • This will tell go to execute the test binary using rr
   • The trace location should be derived from the package path
   • Overriding the output location makes it far easier to determine which trace belongs to which package
   • Otherwise the trace for pkg/foo/bar will be written to ~/.local/share/rr/bar.test-0