Add gojson2subunit script for `go test -json` streams

JUnit XML is the de-facto interchange format for JVM test runners
(Maven Surefire, Gradle, Ant) and many other ecosystems. Both Maven
and Gradle write one XML file per test class into a reports directory,
so the script accepts directories (`-d DIR`) as well as individual
files and walks each directory in lexical order for a deterministic
stream.

Each <testcase> becomes a paired inprogress/terminal subunit packet,
spaced by the testcase's `time` attribute so consumers can recover
the duration. <failure> and <error> children both map to subunit
`fail` (subunit has no separate "error" status that maps cleanly,
and from a consumer's perspective both mean "did not pass"); their
`type`/`message` attributes plus body text are folded into a single
text/plain attachment on the terminal packet.

Add junitxml2subunit script for JUnit XML test reports