fix: Use elements match in gapsBetweenTSDBsAndMetas test

[salvacorts]

What this PR does / why we need it:
This test was sometimes failing with:

Error:      	Not equal: 
        	            	expected: []planner.tsdbGaps{planner.tsdbGaps{tsdbIdentifier:tsdb.SingleTenantTSDBIdentifier{exportTSInSecs:false, TS:time.Time{wall:0x0, ext:62135596800, loc:(*time.Location)(0x3baad[20](https://github.com/grafana/loki/actions/runs/10176790602/job/28150967362#step:5:21))}, From:0, Through:0, Checksum:0x0}, tsdb:common.ClosableForSeries(nil), gaps:[]v1.FingerprintBounds{v1.FingerprintBounds{Min:0x6, Max:0xa}}}, planner.tsdbGaps{tsdbIdentifier:tsdb.SingleTenantTSDBIdentifier{exportTSInSecs:false, TS:time.Time{wall:0x0, ext:62135596801, loc:(*time.Location)(0x3baad20)}, From:0, Through:0, Checksum:0x0}, tsdb:common.ClosableForSeries(nil), gaps:[]v1.FingerprintBounds{v1.FingerprintBounds{Min:0x9, Max:0xa}}}}
        	            	actual  : []planner.tsdbGaps{planner.tsdbGaps{tsdbIdentifier:tsdb.SingleTenantTSDBIdentifier{exportTSInSecs:false, TS:time.Time{wall:0x0, ext:62135596801, loc:(*time.Location)(0x3baad20)}, From:0, Through:0, Checksum:0x0}, tsdb:common.ClosableForSeries(nil), gaps:[]v1.FingerprintBounds{v1.FingerprintBounds{Min:0x9, Max:0xa}}}, planner.tsdbGaps{tsdbIdentifier:tsdb.SingleTenantTSDBIdentifier{exportTSInSecs:false, TS:time.Time{wall:0x0, ext:6[21](https://github.com/grafana/loki/actions/runs/10176790602/job/28150967362#step:5:22)35596800, loc:(*time.Location)(0x3baad20)}, From:0, Through:0, Checksum:0x0}, tsdb:common.ClosableForSeries(nil), gaps:[]v1.FingerprintBounds{v1.FingerprintBounds{Min:0x6, Max:0xa}}}}

Note that the content of both slices is the same but with a different order.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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