net: pass TMPDIR to test client process

Fixes the TestSplice test on Android where the default
TMPDIR (/data/local/tmp) might not be available.

Change-Id: I4f104d11254ba855b1bd2dfa0547d69b7bce4878
Reviewed-on: https://go-review.googlesource.com/c/go/+/170947
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/splice_test.go

@@ -369,6 +369,7 @@ func startSpliceClient(conn Conn, op string, chunkSize, totalSize int) (func(),
 		"GO_NET_TEST_SPLICE_OP=" + op,
 		"GO_NET_TEST_SPLICE_CHUNK_SIZE=" + strconv.Itoa(chunkSize),
 		"GO_NET_TEST_SPLICE_TOTAL_SIZE=" + strconv.Itoa(totalSize),
+		"TMPDIR=" + os.Getenv("TMPDIR"),
 	}
 	cmd.ExtraFiles = append(cmd.ExtraFiles, f)
 	cmd.Stdout = os.Stdout