test: Fix race condition in LogQL test

[bboreham]

What this PR does / why we need it:
Fix a race condition in LogQL test.
The iterators used in TestStepEvaluator_Error were shared across goroutines, leading to a warning from the race detector. Instead, return a new data structure each time.

Also revert #12223 and #12225 which were a previous change to remove the warning, but negatively impact performance.

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/pkg/logql
                                            │ before.txt  │             after.txt             │
                                            │   sec/op    │   sec/op     vs base              │
_RangeVectorIteratorCompare/streaming_agg-8   4.791µ ± 1%   4.793µ ± 3%       ~ (p=0.937 n=6)
_RangeVectorIteratorCompare/batch_agg-8       6.760µ ± 2%   6.589µ ± 1%  -2.53% (p=0.009 n=6)
_RangeVectorIterator-8                        4.855µ ± 1%   4.758µ ± 2%       ~ (p=0.121 n=6)
geomean                                       5.397µ        5.316µ       -1.50%

                                            │  before.txt  │             after.txt              │
                                            │     B/op     │     B/op      vs base              │
_RangeVectorIteratorCompare/streaming_agg-8   3.872Ki ± 0%   3.856Ki ± 0%  -0.40% (p=0.002 n=6)
_RangeVectorIteratorCompare/batch_agg-8       33.99Ki ± 0%   33.97Ki ± 0%  -0.06% (p=0.002 n=6)
_RangeVectorIterator-8                        3.880Ki ± 0%   3.856Ki ± 0%  -0.60% (p=0.002 n=6)
geomean                                       7.993Ki        7.965Ki       -0.35%

                                            │ before.txt │            after.txt             │
                                            │ allocs/op  │ allocs/op   vs base              │
_RangeVectorIteratorCompare/streaming_agg-8   65.00 ± 0%   63.00 ± 0%  -3.08% (p=0.002 n=6)
_RangeVectorIteratorCompare/batch_agg-8       39.00 ± 0%   37.00 ± 0%  -5.13% (p=0.002 n=6)
_RangeVectorIterator-8                        66.00 ± 0%   64.00 ± 0%  -3.03% (p=0.002 n=6)
geomean                                       55.10        53.04       -3.75%

Which issue(s) this PR fixes:
Part of #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• NA Documentation added
• Tests updated
• NA CHANGELOG.md updated

[na--]

Thank you for catching and fixing this! 🙇