cmd/go/internal/test: prepend -test.timeout rather than appending

Tests may accept positional arguments, in which case the -test.timeout
flag must be passed before those arguments.

Fixes #34072

Change-Id: I5b92d7c0edc4f9e1efb63b0733937b76236c0eff
Reviewed-on: https://go-review.googlesource.com/c/go/+/193297
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -572,8 +572,9 @@ func runTest(cmd *base.Command, args []string) {
 	}
 
 	// Pass timeout to tests if it exists.
+	// Prepend rather than appending so that it appears before positional arguments.
 	if testActualTimeout > 0 {
-		testArgs = append(testArgs, "-test.timeout="+testActualTimeout.String())
+		testArgs = append([]string{"-test.timeout=" + testActualTimeout.String()}, testArgs...)
 	}
 
 	// show passing test output (after buffering) with -v flag.

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

@@ -2,12 +2,13 @@
 env GO111MODULE=off
 cd a
 
-# No timeout is passed via 'go test' command.
-go test -v
+# If no timeout is set explicitly, 'go test' should set
+# -test.timeout to its internal deadline.
+go test -v . --
 stdout '10m0s'
 
-# Timeout is passed via 'go test' command.
-go test -v -timeout 30m
+# An explicit -timeout argument should be propagated to -test.timeout.
+go test -v -timeout 30m . --
 stdout '30m0s'
 
 -- a/timeout_test.go --