zfs: the keeper tests' socket directory is the shortest that works

In the test's temporary directory, or /tmp when the socket path would pass 100 characters, so a long TMPDIR cannot fail the bind.

Builds: design 42 zfs-owns-zfs §18, A-37

modules/zfs/cmd/zfs-tools/keeper_test.go

@@ -65,6 +65,17 @@ func TestTheKeeperAnswersOnlyItsClosedListOfCalls(t *testing.T) {
 	}
 }
 
+// shortTempDir is a directory in the test's temporary directory, or in /tmp when a socket in it (`k/k.sock`) would pass 100
+// characters, the most a unix socket path takes, so a long TMPDIR cannot fail the bind.
+func shortTempDir(prefix string) (string, error) {
+	dir, err := os.MkdirTemp("", prefix)
+	if err != nil || len(filepath.Join(dir, "k", "k.sock")) <= 100 {
+		return dir, err
+	}
+	os.RemoveAll(dir)
+	return os.MkdirTemp("/tmp", prefix)
+}
+
 // A-37: only root opens the socket; a root process says and reads through it as through a bus of its own, and with no
 // keeper answering it acts as with the bus down, with a warning (A-8).
 func TestOnlyRootOpensTheKeepersSocketAndANeedlessKeeperIsTheBusDown(t *testing.T) {
@@ -73,7 +84,7 @@ func TestOnlyRootOpensTheKeepersSocketAndANeedlessKeeperIsTheBusDown(t *testing.
 	k := &Keeper{Emit: func(string, any) error { return nil }, States: st,
 		Call:   func(string, any) (json.RawMessage, error) { return nil, errors.New("down") },
 		PeerOf: func(net.Conn) (int, error) { return uid, nil }}
-	dir, err := os.MkdirTemp("", "zk")
+	dir, err := shortTempDir("zk")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -148,7 +159,7 @@ func TestOnlyRootOpensTheKeepersSocketAndANeedlessKeeperIsTheBusDown(t *testing.
 // A-37: a root process whose keeper does not answer acts as with the bus down and says so, once, in a warning, for the
 // hand acts, the forgetting of applied keys and the pause alike.
 func TestAKeeperThatDoesNotAnswerIsWarnedOnce(t *testing.T) {
-	dir, err := os.MkdirTemp("", "keeper-down")
+	dir, err := shortTempDir("kd")
 	if err != nil {
 		t.Fatal(err)
 	}