test: Data race updates for memchunk tests

[paul1r]

What this PR does / why we need it:
Multiple instances of a data race related to the noopStreamPipeline variable. This was resolved by creating a new one per test, as opposed to sharing a single instance between all tests.

After updates:

go test -race -count=1 .   -v -timeout 0
PASS
ok  	github.com/grafana/loki/v3/pkg/chunkenc	776.725s

Which issue(s) this PR fixes:
Relates to #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR