40600: storage/engine: return WriteIntentError for intents in uncertainty intervals r=petermattis a=nvanbenschoten

This commit fixes the most common failure case in all of the following Jepsen failures. I'm not closing them here though, because they also should be failing due to #36431.
Would fix #37394.
Would fix #37545.
Would fix #37930.
Would fix #37932.
Would fix #37956.
Would fix #38126.
Would fix #38763.

Before this fix, we were not considering intents in a scan's uncertainty interval to be uncertain. This had the potential to cause stale reads because an unresolved intent doesn't indicate that its transaction hasn’t been committed and isn’t a causal ancestor of the scan. This was causing the `jepsen/multi-register` tests to fail, which I had previously incorrectly attributed entirely to #36431.

This commit fixes this by returning `WriteIntentError`s for intents when they are above the read timestamp of a scan but below the max timestamp of a scan. This could have also been fixed by returning `ReadWithinUncertaintyIntervalError`s in this situation. Both would eventually have the same effect, but it seems preferable to kick off concurrency control immediately in this situation and only fall back to uncertainty handling for committed values. If the intent ends up being aborted, this could allow the read to avoid moving its timestamp.

This commit will need to be backported all the way back to v2.0.

Release note (bug fix): Consider intents in a read's uncertainty interval to be uncertain just as if they were committed values. This removes the potential for stale reads when a causally dependent transaction runs into the not-yet resolved intents from a causal ancestor.

40603: make: pass TESTFLAGS to roachprod-stress, not GOFLAGS r=petermattis a=nvanbenschoten

Passing the testflags through the GOFLAGS env var was causing the following
error:
```
stringer -output=pkg/sql/opt/rule_name_string.go -type=RuleName pkg/sql/opt/rule_name.go pkg/sql/opt/rule_name.og.go
stringer: go [list -f {{context.GOARCH}} {{context.Compiler}} -tags= -- unsafe]: exit status 1: go: parsing $GOFLAGS: non-flag "storage.test"
Makefile:1496: recipe for target 'pkg/sql/opt/rule_name_string.go' failed
make: *** [pkg/sql/opt/rule_name_string.go] Error 1
make: *** Waiting for unfinished jobs....
```

Release note: None

Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>