tstest/integration: deflake TestCollectPanic

Two issues caused TestCollectPanic to flake:

1. ETXTBSY: The test exec'd the tailscaled binary directly without
   going through StartDaemon/awaitTailscaledRunnable, so it lacked
   the retry loop that other tests use to work around a mysterious
   ETXTBSY on GitHub Actions.

2. Shared filch files: The test didn't pass --statedir or TS_LOGS_DIR,
   so all parallel test instances wrote panic logs to the shared system
   state directory (~/.local/share/tailscale). Concurrent runs would
   clobber each other's filch log files, causing the second run to not
   find the panic data from the first.

Fix both by adding awaitTailscaledRunnable before the first exec, and
passing --statedir and TS_LOGS_DIR to isolate each test's log files,
matching what StartDaemon does.

It now passes x/tools/cmd/stress.

Fixes #15865

Change-Id: If18b9acf8dbe9a986446a42c5d98de7ad8aae098
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/integration_test.go

@@ -200,23 +200,34 @@ func TestExpectedFeaturesLinked(t *testing.T) {
 }
 
 func TestCollectPanic(t *testing.T) {
-	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/15865")
 	tstest.Shard(t)
 	tstest.Parallel(t)
 	env := NewTestEnv(t)
 	n := NewTestNode(t, env)
 
-	cmd := exec.Command(env.daemon, "--cleanup")
+	// Wait for the binary to be executable, working around a
+	// mysterious ETXTBSY on GitHub Actions.
+	// See https://github.com/tailscale/tailscale/issues/15868.
+	if err := n.awaitTailscaledRunnable(); err != nil {
+		t.Fatal(err)
+	}
+
+	logsDir := t.TempDir()
+	cmd := exec.Command(env.daemon, "--cleanup", "--statedir="+n.dir)
 	cmd.Env = append(os.Environ(),
 		"TS_PLEASE_PANIC=1",
 		"TS_LOG_TARGET="+n.env.LogCatcherServer.URL,
+		"TS_LOGS_DIR="+logsDir,
 	)
 	got, _ := cmd.CombinedOutput() // we expect it to fail, ignore err
 	t.Logf("initial run: %s", got)
 
 	// Now we run it again, and on start, it will upload the logs to logcatcher.
-	cmd = exec.Command(env.daemon, "--cleanup")
-	cmd.Env = append(os.Environ(), "TS_LOG_TARGET="+n.env.LogCatcherServer.URL)
+	cmd = exec.Command(env.daemon, "--cleanup", "--statedir="+n.dir)
+	cmd.Env = append(os.Environ(),
+		"TS_LOG_TARGET="+n.env.LogCatcherServer.URL,
+		"TS_LOGS_DIR="+logsDir,
+	)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		t.Fatalf("cleanup failed: %v: %q", err, out)
 	}