*: evaluate uses of `skip.Stress()`

With the RBE-based CI and nightlies, this variable will not be set in CI, which means that `skip.Stress()` will always return `false`. The following uses of it should be evaluated and maybe replaced with alternatives.

```
cockroach$ rg skip.Stress
pkg/sql/plan_opt_test.go
118: if !skip.Stress() {

pkg/sql/mvcc_statistics_update_job_test.go
211: if skip.Stress() {

pkg/ccl/streamingccl/replicationtestutils/testutils.go
494: if skip.Stress() || util.RaceEnabled {

pkg/ccl/kvccl/kvtenantccl/upgradeinterlockccl/local_test_util_test.go
113: if skip.Stress() {

```

See also:
#123907
#123908

Jira issue: CRDB-38598