Check the tool runtime's own code before it merges, every test on a bus of its own (hq ADR 0237)

A merge-check.sh, the repository's layer of the mesh's merge check (mesh/repo-check):
format, vet, and node-tools' suite under the race detector. The tests shared one bus and
had to run one package at a time; like the controller's (#97), each now starts a server
of its own at the release go.mod pins, held to the catalogue's bus image by a test. What
cannot run in the check — bundles that need @novox/mesh-sdk — is said as not tested.

node-tools/internal/meshtest/meshtest.go

@@ -1,9 +1,13 @@
 // Package meshtest raises what the controller would, for tests against a real bus: the ASSIGNMENTS
 // and EVENTS streams, memberships issued by hand, and a fixture's path.
 //
-// **The packages share one bus, so run them one at a time: `go test -p 1 ./...`.** Each test raises
-// the streams afresh, and the console discovers every runtime that announces itself on the bus
-// (novox/hq ADR 0197) — a runtime from another package's test is, correctly, found.
+// **Every test has a bus of its own** (novox/hq ADR 0237, the controller's internal/testbus): a server
+// linked in at the release go.mod pins — the release the mesh runs, held so by a test beside this one —
+// started for the one test and gone after it. The packages shared one bus and had to run one at a time:
+// each test raised the streams afresh, and the console discovers every runtime that announces itself on
+// the bus (ADR 0197), so a runtime from another package's test was, correctly, found. Now the suite runs
+// as Go runs it, in parallel and under the race detector. A person may still point a run at a bus of
+// their own with MESH_TEST_NATS_EXTERNAL=1 and MESH_TEST_NATS; then the run is theirs to serialise.
 package meshtest
 
 import (
@@ -12,24 +16,57 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
+	"github.com/nats-io/nats-server/v2/server"
 	"github.com/nats-io/nats.go"
 
 	"github.com/novox/mesh-tools/node-tools/internal/bus"
 )
 
-// URL is the test bus, or the test is skipped.
+// Version is the release of the server the tests run.
+const Version = server.VERSION
+
+// URL is the bus of this test alone: started at its first ask, the same one at every ask after — a test
+// that dials twice, or hands the address to the code it tests, reaches one bus — and shut down when the
+// test ends.
 func URL(t *testing.T) string {
 	t.Helper()
-	url := os.Getenv("MESH_TEST_NATS")
-	if url == "" {
-		t.Skip("MESH_TEST_NATS unset")
+	if os.Getenv("MESH_TEST_NATS_EXTERNAL") == "1" {
+		if url := os.Getenv("MESH_TEST_NATS"); url != "" {
+			return url
+		}
+		t.Fatal("MESH_TEST_NATS_EXTERNAL=1 and MESH_TEST_NATS names no bus")
 	}
+	if url, ok := buses.Load(t); ok {
+		return url.(string)
+	}
+	opts := &server.Options{Host: "127.0.0.1", Port: server.RANDOM_PORT, JetStream: true, StoreDir: t.TempDir(),
+		NoLog: true, NoSigs: true}
+	s, err := server.NewServer(opts)
+	if err != nil {
+		t.Fatalf("a bus for this test could not be made: %v", err)
+	}
+	go s.Start()
+	if !s.ReadyForConnections(30 * time.Second) {
+		s.Shutdown()
+		t.Fatal("a bus for this test did not come up within 30s")
+	}
+	url := s.ClientURL()
+	buses.Store(t, url)
+	t.Cleanup(func() {
+		buses.Delete(t)
+		s.Shutdown()
+		s.WaitForShutdown()
+	})
 	return url
 }
 
+// buses are the running tests' buses, by test.
+var buses sync.Map
+
 // Mesh is the controller's job, done by hand.
 type Mesh struct {
 	nc *nats.Conn