roachtest: add import/mixed-version roachtest

Before supporting mixed-versions runs of IMPORT this test would fail
with:
```
importing fixture: importing table history: pq: version mismatch in flow request: 42; this node accepts 35 through 36
```

It now succeeds. Note that this test exercises that IMPORT can run in a
mixed-version cluster, but not during an upgrade of nodes in the
cluster.

Release note: None

pkg/cmd/roachtest/import.go

@@ -168,3 +168,54 @@ func registerImportTPCH(r *testRegistry) {
 		})
 	}
 }
+
+func successfulImportStep(warehouses, nodeID int) versionStep {
+	return func(ctx context.Context, t *test, u *versionUpgradeTest) {
+		u.c.Run(ctx, u.c.Node(nodeID), tpccImportCmd(warehouses))
+	}
+}
+
+func runImportMixedVersion(
+	ctx context.Context, t *test, c *cluster, warehouses int, predecessorVersion string,
+) {
+	// An empty string means that the cockroach binary specified by flag
+	// `cockroach` will be used.
+	const mainVersion = ""
+	roachNodes := c.All()
+
+	t.Status("starting csv servers")
+
+	u := newVersionUpgradeTest(c,
+		uploadAndStartFromCheckpointFixture(roachNodes, predecessorVersion),
+		waitForUpgradeStep(roachNodes),
+		preventAutoUpgradeStep(1),
+
+		// Upgrade some of the nodes.
+		binaryUpgradeStep(c.Node(1), mainVersion),
+		binaryUpgradeStep(c.Node(2), mainVersion),
+
+		successfulImportStep(warehouses, 1 /* nodeID */),
+	)
+	u.run(ctx, t)
+}
+
+func registerImportMixedVersion(r *testRegistry) {
+	r.Add(testSpec{
+		Name:  "import/mixed-versions",
+		Owner: OwnerBulkIO,
+		// Mixed-version support was added in 21.1.
+		MinVersion: "v21.1.0",
+		Cluster:    makeClusterSpec(4),
+		Run: func(ctx context.Context, t *test, c *cluster) {
+			predV, err := PredecessorVersion(r.buildVersion)
+			if err != nil {
+				t.Fatal(err)
+			}
+			warehouses := 100
+			if local {
+				warehouses = 10
+			}
+			runImportMixedVersion(ctx, t, c, warehouses, predV)
+		},
+	})
+}

pkg/cmd/roachtest/registry.go

@@ -41,6 +41,7 @@ func registerTests(r *testRegistry) {
 	registerGossip(r)
 	registerHibernate(r)
 	registerHotSpotSplits(r)
+	registerImportMixedVersion(r)
 	registerImportTPCC(r)
 	registerImportTPCH(r)
 	registerInconsistency(r)