cmd/test2json: document missing "skip" action

Change-Id: I906e61170279f0647598e2fd4fa931aac1b69288
GitHub-Last-Rev: f6df43e8e10e3b032a67490611c0ba5ad8e948df
GitHub-Pull-Request: golang/go#24517
Reviewed-on: https://go-review.googlesource.com/102396Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/test2json/main.go

@@ -48,6 +48,7 @@
 //	bench  - the benchmark printed log output but did not fail
 //	fail   - the test or benchmark failed
 //	output - the test printed output
+//	skip   - the test was skipped or the package contained no tests
 //
 // The Package field, if present, specifies the package being tested.
 // When the go command runs parallel tests in -json mode, events from