mixedversion: add RunE method to Test

This patch adds a `RunE` method to `Test`. Unlike the existing `Run` function,
it will not immediately fatal the test if an error is encountered and will
instead return the error along with the test plan so that callers may decide
how to handle the error.

Release note: None

Author: andyyang890

pkg/cmd/roachtest/roachtestutil/mixedversion/mixedversion.go

@@ -834,21 +834,29 @@ func (t *Test) Workload(
 	return t.BackgroundCommand(fmt.Sprintf("%s workload", name), node, runCmd)
 }
 
-// Run runs the mixed-version test. It should be called once all
+// Run is like RunE, except it fatals the test if any error occurs.
+func (t *Test) Run() {
+	_, err := t.RunE()
+	if err != nil {
+		t.rt.Fatal(err)
+	}
+}
+
+// RunE runs the mixed-version test. It should be called once all
 // startup, mixed-version, and after-test hooks have been declared. A
 // test plan will be generated (and logged), and the test will be
-// carried out.
-func (t *Test) Run() {
+// carried out. A non-nil plan will be returned unless planning fails.
+func (t *Test) RunE() (*TestPlan, error) {
 	plan, err := t.plan()
 	if err != nil {
-		t.rt.Fatal(err)
+		return nil, err
 	}
 
 	t.logger.Printf("mixed-version test:\n%s", plan.PrettyPrint())
 
 	if override := os.Getenv(dryRunEnv); override != "" {
 		t.logger.Printf("skipping test run in dry-run mode")
-		return
+		return plan, nil
 	}
 
 	// Mark the deployment mode and versions, so they show up in the github issue. This makes
@@ -857,8 +865,10 @@ func (t *Test) Run() {
 	t.rt.AddParam("mvtVersions", formatVersions(plan.Versions()))
 
 	if err := t.run(plan); err != nil {
-		t.rt.Fatal(err)
+		return plan, err
 	}
+
+	return plan, nil
 }
 
 func (t *Test) run(plan *TestPlan) error {