database/sql: flaky test TestTxContextWait

https://storage.googleapis.com/go-build-log/779cc599/linux-amd64-race_8fa25f3c.log

```
--- FAIL: TestTxContextWait (0.02s)
	sql_test.go:395: context deadline exceeded
FAIL
FAIL	database/sql	0.924s
```

(see on trybots of https://go-review.googlesource.com/c/36219/)

More of #18115?

/cc @rsc 