Add duration filter unit test to logql parser

[btaani]

What this PR does / why we need it:
This PR adds a unit test to pkg/logql/syntax/parser_test.go that addresses the support of promql-style duration filter.

Which issue(s) this PR fixes:
Fixes #4706

Special notes for your reviewer:
The issue itself was fixed here, this PR only adds the corresponding unit test.

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[dannykopping]

LGTM, thanks for the contribution @btaani 👍

[CLAassistant]

All committers have signed the CLA.

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
-        distributor	-0.3%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
+               loki	0%