Revert "cmd/go/internal/test: wrap os.Stdout always"

This reverts CL 400877.

Reason for revert: broke TestScript/test_output_wait and the
regression test for #18153 on certain builders.

Fixes #52461.

Change-Id: I98627ce5e22088b0784be502c459480c41ba353a
Reviewed-on: https://go-review.googlesource.com/c/go/+/401494
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

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

@@ -1261,11 +1261,7 @@ func (c *runCache) builderRunTest(b *work.Builder, ctx context.Context, a *work.
 		return nil
 	}
 
-	// The os/exec package treats an *os.File differently to an io.Writer.
-	// Embed os.Stdout in an io.Writer struct so that we get the same
-	// behavior regardless of whether we wrap it below.
-	// See golang.org/issue/24050
-	var stdout io.Writer = struct{ io.Writer }{os.Stdout}
+	var stdout io.Writer = os.Stdout
 	var err error
 	if testJSON {
 		json := test2json.NewConverter(lockedStdout{}, a.Package.ImportPath, test2json.Timestamp)