server: fix StartedDefaultTestTenant in an error case

Previously, if `TestServer.Start` returned an error while the default
test tenant wasn't disabled, we'd incorrectly consider that we had
started the default test tenant. This, then, could lead to some problems
(e.g. trying to access non-existing tenant which could lead to a crash).
This is fixed by checking whether we have test tenants.

Release note: None

pkg/server/testserver.go

@@ -1714,7 +1714,7 @@ func (ts *TestServer) ExecutorConfig() interface{} {
 
 // StartedDefaultTestTenant is part of the serverutils.TestServerInterface.
 func (ts *TestServer) StartedDefaultTestTenant() bool {
-	return !ts.cfg.DisableDefaultTestTenant
+	return len(ts.testTenants) > 0
 }
 
 // TenantOrServer is part of the serverutils.TestServerInterface.