cmd/dist: test -json emits some non-JSON lines for packages without tests

[dmitshur]

When the -json flag is used with go tool dist test (added in #37486), it's expected to only write JSON lines.

There seems to be a path where some non-JSON is written to stdout too, which is a bug:

$ go tool dist test -json -run=crypto/...:gofips140
{"Time":"2025-10-23T12:36:35.439236-04:00","Action":"start","Package":"crypto:gofips140"}
{"Time":"2025-10-23T12:36:35.443492-04:00","Action":"start","Package":"crypto/aes:gofips140"}
{"Time":"2025-10-23T12:36:35.445535-04:00","Action":"start","Package":"crypto/cipher:gofips140"}
[...]
?       crypto/internal/boring/bbig    [no test files]
?       crypto/internal/boring/sig    [no test files]
?       crypto/internal/cryptotest    [no test files]
[...]

[gabyhelp]

Related Issues

• cmd/test2json: test output without trailing newline missing final status #57172 (closed)
• test: add JSON output support to the run.go program #56844 (closed)
• cmd/dist: runPending doesn't JSON-encode skips in -json mode #61557 (closed)
• stdout messes up `go test --json` #36576 (closed)
• cmd/go: go test -json has non-JSON printed to stdout when build fails #35169 (closed)
• cmd/go: go test -json outputs too verbose #40588

Related Code Changes

• cmd/dist: add -json flag
• cmd/dist: add JSON output support for some tests
• cmd/dist: add -json flag
• cmd/dist: handle -json flag in runPending (clean up)

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