sql: don't use t.Parallel() in TestQueryCache

We started running sub-tests of `TestQueryCache` in parallel long time
ago in b1ebe33. Later on we disabled
the parallelism under stress in 1cf1edc.

Each sub-test starts an in-memory server, and we have about 15
sub-tests, so this could be problematic in "heavy" configs. Recently we
saw some flakes after a change to the underlying test infra ("noisy
neighbor" was introduced), and even though that change has been
reverted, I don't see why we should be using `t.Parallel()` for this
particular test at all - it's almost the only one within `sql` folder
that does this. Thus, this commit removes the parallelization
altogether. On my laptop I saw the time to run in a regular config
increase from 1.5s to 4.5s, and we definitely have slower tests, so it
shouldn't matter in practice.

Release note: None

Author: yuzefovich