roachtest: automate and de-flake version-upgrade

The recent changes to this test seem to have uncovered a latent [bug],
as a result of which the test was skipped. This commit lets the test
start from v19.2 fixtures (instead of v19.1 before) which experimentally
were verified to not exhibit the bug, so the test is unskipped.

While I was there, I automated the base version discovery of this test.
On a branch at version X, the test will now use the fixtures for the
last major release preceding X. We'll occasionally have to bake new
fixtures and update the mapping in PredecessorVersion() but this is
better than having the test rot on older branches whenever we update
it for a new dev cycle.

The root cause of the bug will be fixed separately; this is tracked in
issue #44732.

[bug]: #47235 (comment)

Release note: None

pkg/cmd/roachtest/acceptance.go

@@ -46,8 +46,13 @@ func registerAcceptance(r *testRegistry) {
 		{name: "status-server", fn: runStatusServer},
 		{
 			name: "version-upgrade",
-			fn:   runVersionUpgrade,
-			skip: "skipped due to flakiness",
+			fn: func(ctx context.Context, t *test, c *cluster) {
+				predV, err := PredecessorVersion(r.buildVersion)
+				if err != nil {
+					t.Fatal(err)
+				}
+				runVersionUpgrade(ctx, t, c, predV)
+			},
 			// This test doesn't like running on old versions because it upgrades to
 			// the latest released version and then it tries to "head", where head is
 			// the cockroach binary built from the branch on which the test is

pkg/cmd/roachtest/versionupgrade.go

@@ -80,21 +80,20 @@ const (
 	headVersion = "HEAD"
 )
 
-func runVersionUpgrade(ctx context.Context, t *test, c *cluster) {
+func runVersionUpgrade(ctx context.Context, t *test, c *cluster, predecessorVersion string) {
 	// This is ugly, but we can't pass `--encrypt=false` to old versions of
 	// Cockroach.
 	//
 	// TODO(tbg): revisit as old versions are aged out of this test.
 	c.encryptDefault = false
 
-	const baseVersion = "19.1.5"
 	u := newVersionUpgradeTest(c, versionUpgradeTestFeatures,
 		// Load baseVersion fixture. That fixture's cluster version may be
 		// at the predecessor version, so add a waitForUpgradeStep to make
 		// sure we're actually running at baseVersion before moving on.
 		//
 		// See the comment on createCheckpoints for details on fixtures.
-		uploadAndStartFromCheckpointFixture(baseVersion),
+		uploadAndStartFromCheckpointFixture(predecessorVersion),
 		waitForUpgradeStep(),
 
 		// NB: before the next step, cluster and binary version equals baseVersion,