cmd/go: add document -json in testflag

Fixes #47628

Change-Id: I2776fbc22d8a73ca7adc2cf7ad85669d57cc7eae
GitHub-Last-Rev: 826907b079
GitHub-Pull-Request: golang/go#47683
Reviewed-on: https://go-review.googlesource.com/c/go/+/341991
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/alldocs.go

@@ -2658,6 +2658,10 @@
 // 	-failfast
 // 	    Do not start new tests after the first test failure.
 //
+// 	-json
+// 	    Log verbose output and test results in JSON. This presents the
+// 	    same information as the -v flag in a machine-readable format.
+//
 // 	-list regexp
 // 	    List tests, benchmarks, or examples matching the regular expression.
 // 	    No tests, benchmarks or examples will be run. This will only

src/cmd/go/internal/test/test.go

@@ -242,6 +242,10 @@ control the execution of any test:
 	-failfast
 	    Do not start new tests after the first test failure.
 
+	-json
+	    Log verbose output and test results in JSON. This presents the
+	    same information as the -v flag in a machine-readable format.
+
 	-list regexp
 	    List tests, benchmarks, or examples matching the regular expression.
 	    No tests, benchmarks or examples will be run. This will only