read "go test" output from stdin

In Kubernetes, we run fairly complex commands to execute our unit tests. gotestsum is then used for post-processing with `--raw-command cat `.

It might be possible to turn those complex commands into a script (I'm trying...) but the shell quoting is nasty.

Would you accept a patch that makes `gotestsum` read input from stdin?

Start by tracing how the existing `--raw-command cat ` flow receives and processes test output, then inspect its command-line parsing and related tests. Done means gotestsum can accept equivalent `go test` output from stdin, with tests covering the new input path.

Written by the indexing model from the issue text.