test: Increase buffer size to prevent timeouts in tests

[paul1r]

What this PR does / why we need it:
A recent PR caused multiple occurrences of timeouts in this particular test. This PR increases the buffer size to work around this.

Which issue(s) this PR fixes:
Fixes #

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
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• 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

[paul1r]

go test -race -count=500 -run Test_Push/batches_push_requests 
PASS
ok  	github.com/grafana/loki/v3/pkg/pattern/aggregation	2.173s

[trevorwhitney]

thanks Paul!