cmd/go: drop FailedBuild field if gotestjsonbuildtext=1

go test -json has two new effects in Go 1.24: it implies go build
-json, and it adds a FailedBuild field in test events. For
compatibility, CL 629335 added gotestjsonbuildtext=1, which disables
the implicit go build -json, but that CL didn't affect the FailedBuild
field. In principle this shouldn't matter because it's just another
JSON field, but just so we don't have to worry about some intermediate
behavior, this CL makes gotestjsonbuildtext=1 disable the FailedBuild
field as well.

Updates #62067
Updates #70402

Change-Id: I006d1ea0468980ee2564495324e8b4ed082898af
Reviewed-on: https://go-review.googlesource.com/c/go/+/635899
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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

@@ -1009,7 +1009,13 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 					json.Exited(err)
 					json.Close()
 				}()
-				json.SetFailedBuild(perr.Desc())
+				if gotestjsonbuildtext.Value() == "1" {
+					// While this flag is about go build -json, the other effect
+					// of that change was to include "FailedBuild" in the test JSON.
+					gotestjsonbuildtext.IncNonDefault()
+				} else {
+					json.SetFailedBuild(perr.Desc())
+				}
 				stdout = json
 			}
 			fmt.Fprintf(stdout, "FAIL\t%s [setup failed]\n", p.ImportPath)
@@ -1437,7 +1443,11 @@ func (r *runTestActor) Act(b *work.Builder, ctx context.Context, a *work.Action)
 	if a.Failed != nil {
 		// We were unable to build the binary.
 		if json != nil && a.Failed.Package != nil {
-			json.SetFailedBuild(a.Failed.Package.Desc())
+			if gotestjsonbuildtext.Value() == "1" {
+				gotestjsonbuildtext.IncNonDefault()
+			} else {
+				json.SetFailedBuild(a.Failed.Package.Desc())
+			}
 		}
 		a.Failed = nil
 		fmt.Fprintf(stdout, "FAIL\t%s [build failed]\n", a.Package.ImportPath)

src/cmd/go/testdata/script/test_json_build.txt

@@ -58,8 +58,9 @@ stderr '# m/builderror \[m/builderror.test\]\n'
 stderr 'builderror'${/}'main_test.go:3:11: undefined: y\n'
 stdout '"Action":"start","Package":"m/builderror"'
 stdout '"Action":"output","Package":"m/builderror","Output":"FAIL\\tm/builderror \[build failed\]\\n"'
-stdout '"Action":"fail","Package":"m/builderror","Elapsed":.*,"FailedBuild":"m/builderror \[m/builderror\.test\]"'
-
+stdout '"Action":"fail","Package":"m/builderror","Elapsed":[0-9.]+\}'
+# FailedBuild should NOT appear in the output in this mode.
+! stdout '"FailedBuild"'
 
 -- go.mod --
 module m